arch/arm/am67: Add rptun IPC with the Linux A53 (remoteproc/rpmsg)

[gemstonedevteam]

Four commits: the drivers/rptun resource-table sizing change, the AM67 rptun
lowerhalf, the RAT sliding window, an nxstyle fix to a file this PR touches, and
documentation.

Summary

• Adds the rptun lowerhalf connecting the R5F to Linux remoteproc over NAVSS
  mailbox0_cluster3 (R5F TX on FIFO 0 → Linux GIC SPI 109; Linux TX on
  FIFO 1 → VIM IRQ 116).
• The mailbox ISR is a strict top half: drain the FIFO, W1C the status, EOI.
  OpenAMP delivery is deferred to HPWORK, because the rpmsg rx stack takes
  mutexes and allocates, neither of which is legal in interrupt context.
• The resource table publishes two vdevs (rpmsg and virtio-net) with all vring
  device addresses FW_RSC_ADDR_ANY: Linux remoteproc allocates each vring
  from the R5F DMA pool and writes the chosen address back before releasing
  the core; fixed addresses outside the pool are rejected.
• drivers/rptun: rptun_do_start() installed the resource table with a
  hardcoded sizeof(struct rptun_rsc_s), truncating extended tables that
  carry more than the single rpmsg vdev. An optional get_rsc_size() op is
  added with an RPTUN_GET_RSC_SIZE macro that falls back to the old value,
  so drivers using the standard table are unaffected.
• The RAT sliding window addresses a 32-bit R5F against a 64-bit peer: J722S
  maps DDR beyond the first 2 GB at 0x8_8000_0000, outside the R5F's
  addressable space, while Linux freely posts virtio buffers there. Region 0
  is dedicated as a 16 MB window at 0xFE000000, re-aimed per access, with
  the window range MPU-mapped Non-cacheable.
• MPU: two Non-cacheable regions cover the shared IPC memory. The R5F is not
  hardware-coherent with the A53; with cacheable mappings NuttX reads stale
  DRIVER_OK/vring state and the handshake hangs.

Impact

• Is new feature added? YES — rptun/rpmsg IPC for AM67.
• Impact on user? NO — the drivers/rptun change is an optional op with a
  fallback to the previous behaviour; existing rptun drivers are unaffected.
• Impact on build? NO.
• Impact on hardware? YES — arch/arm/am67 and board t3-gem-o1.
• Impact on documentation? YES — provided in this PR.
• Impact on security? NO.
• Impact on compatibility? NO — RPTUN_GET_RSC_SIZE falls back to
  sizeof(struct rptun_rsc_s) when a driver does not implement it.
• Anything else? Per rule 1.7.9, the drivers/rptun sizing change and the AM67
  lowerhalf are bundled deliberately: the lowerhalf's extended two-vdev table
  is truncated without it, so neither is functional alone. This is not a
  breaking change.

Testing

Build Host(s): Pardus GNU/Linux 25 (Debian 13 base), x86_64, host gcc 14
Toolchain:     arm-none-eabi-gcc 14.2.1 20241119 (15:14.2.rel1-1)
Target:        arm / TI AM67 (J722S) Cortex-R5F, board t3-gem-o1:nsh
nuttx-apps:    5a7ab4200
Loaded on the main-domain R5F by Linux remoteproc; console on UART-MAIN1.

Testing logs before change: no IPC support on this arch.

The version string in the logs is from the tested build; the commits were
GPG-signed afterwards, which rewrites hashes. The code is unchanged.

Testing logs after change:

make distclean && ./tools/configure.sh t3-gem-o1:nsh && make -j
  -> nuttx ELF 587972 bytes, 0 compiler warnings

Console:
nsh> ls /dev/rptun
/dev/rptun:
 r5f
nsh> 
nsh> ls /dev/rptun
/dev/rptun:
 r5f
nsh>

Test plan result:
console_alive            PASS  prompt responsive
version                  PASS  NuttX version 0.0.0 488ed91bec Sep 10 2026 13:49:52 t3-gem-o1:nsh
procfs                   PASS  procfs readable
tasks                    PASS  3 task lines
rptun_node               PASS  /dev/rptun:  r5f
linux_remoteproc         PASS  remoteproc3 running, fw=j722s-main-r5f0_0-fw, vdevs=virtio1,virtio2
linux_rpmsg_channels     PASS  2 channel(s): virtio1.rpmsg_ctrl.0.0 virtio1.rpmsg_ns.53.53

PASS=7
make distclean && ./tools/configure.sh t3-gem-o1:nsh && make -j
  -> nuttx ELF 587972 bytes, 0 compiler warnings

Console:
nsh> ls /dev/rptun
/dev/rptun:
 r5f
nsh> 
nsh> ls /dev/rptun
/dev/rptun:
 r5f
nsh>

Test plan result:
console_alive            PASS  prompt responsive
version                  PASS  NuttX version 0.0.0 488ed91bec Sep 10 2026 13:49:52 t3-gem-o1:nsh
procfs                   PASS  procfs readable
tasks                    PASS  3 task lines
rptun_node               PASS  /dev/rptun:  r5f
linux_remoteproc         PASS  remoteproc3 running, fw=j722s-main-r5f0_0-fw, vdevs=virtio1,virtio2
linux_rpmsg_channels     PASS  2 channel(s): virtio1.rpmsg_ctrl.0.0 virtio1.rpmsg_ns.53.53

PASS=7

PR verification Self-Check

• This PR introduces only one functional change.
• I have updated all required description fields above.
• My PR adheres to Contributing Guidelines and Documentation.
• My PR is still work in progress (not ready for review).
• My PR is ready for review and can be safely merged into a codebase.

[github-actions]

MemBrowse Memory Report

No memory changes detected for:

• arduino-mega2560
• esp32-devkitc
• hifive1-revb
• mirtoo
• qemu-armv8a
• qemu-intel64
• rx65n-rsk2mb
• s698pm-dkit
• stm32-nucleo-f103rb

[acassis]

@gemstonedevteam please rebase