CMake: define DATA_IN_D2_SRAM so SystemInit enables the D2 SRAM blocks

[zeroisnan]

Fixes #712.

The CMake build never defined DATA_IN_D2_SRAM, so the RCC->AHB2ENR write in
SystemInit() was compiled out and the D2 AHB SRAM blocks were left disabled.
That is where DMA_BUFFER_MEM_SECTION puts the audio, ADC, MIDI and USB-host DMA
buffers. The Makefile has passed the define since #390.

system_stm32h7xx.c is built by CMSIS_DEVICE_H7 rather than daisy, so the
define goes on that target. PRIVATE matches the Makefile, which passes it when
building libDaisy and not to the application.

Verified on examples/AudioPassthru with arm-none-eabi-gcc 13.2.1, Release:
AHB2ENR |= 0xE0000000 is now present in the CMake build's SystemInit (inlined
into Reset_Handler by LTO), matching the Makefile build. Costs 24 bytes of flash.
cmake --build build --target examples builds clean.

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