soc: st: stm32: h7rs: Add memory region to MPU region list

Add ethernet DMA buffer/descriptor region (sram2) and read only
flash region 0x08FFF800 with unique device ID registers to MPU region
list. The unique device ID is used to create a random mac address by
the ethernet driver. Ethernet DMA buffer/descriptor memory section is
also added to linker script.

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