samples: fs: fat_fs: support on stm32h747i-disco board

This patch allow demonstrating fat_fs sample application on
stm32h747i-disco  board with SD card.

Build fs_sample using following command

west build -p always -b stm32h747i_disco_m7 \
	samples/subsys/fs/fs_sample

Signed-off-by: Murali Karicheri <murali.karicheri@sandc.com>