STM32H7 support for EMMC cards

[MattKow-Firefly]

I was testing an EMMC adapter card (has an EMMC chip, plugs in to a micro-SD slot) with PX4 1.15 and it was not initializing the EMMC card. I traced the issue to the NuttX function static int mmcsd_read_csd(FAR struct mmcsd_state_s *priv) For STM32H7 the alignment for DMA should be 32 byte, not 16 byte. I made changes locally and built and debugged it, and it works now. I will make a Pull Request with the fix.