Set CONFIG_STM32H7_DTCMEXCLUDE=y on the four STM32H7 ARK boards
(fmu-v6x, fmu-v6s, fpv, pi6x) so DTCM is no longer part of the kernel
heap. Stacks and dynamic buffers can otherwise land in DTCM, which AHB
DMA masters (SDMMC IDMA, MDMA, BDMA, DMA1/2) cannot access, causing file
operations to fail with EIO/ETIMEDOUT.
Refs PX4/PX4-Autopilot#27242.