boards: st: nucleo_h7s3l8 & stm32h7s78_dk: Fix ext_mem MPU type

Change the zephyr,memory-attr of the ext_memory / ext_flash_mem node to
ATTR_MPU_FLASH because ATTR_MPU_IO does not allow unaligned memory access.

Signed-off-by: Thomas Decker <decker@jb-lighting.de>