Fix STM32 UART7/8 kconfig names and UART DMA dependencies.
FAT32: FMUv2 workaround for DMA to FAT

This is a hackish workaround to prevent FAT filesystem corruption due
to FAT IO not being aligned on a 4 byte boundary for DMA.

This fix should not be pushed upstream - it is just until we have a
better fix for FAT DMA