boards: stm32f7/stm32h7: Configure external SDRAM region with RAM attribute

On M7 based STM32 boards, external SDRAM may be present and mapped in the
area 0xC0000000-0xDFFFFFFF.
According to ARMv7-M Architecture Reference Manual chapter B3.1
(table B3-1), this area is specified as Device Memory Type, with the
consequence that all accesses should be naturally aligned, otherwise
a hard fault will be triggered whatever UNALIGN_TRP status in the CCR reg.

To avoid this issue, when available, configure external SDRAM memory
regions as a MPU with RAM attribute.

Fixes #54670

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>