soc: renode: cortex_r8_virtual: overhaul MPU regions

Apply the same modifications made to the ZynqMP's memory
regions to the cortex_r8_virtual SoC which was mainlined
while the fixes for the ZynqMP were being developed
(minus the OCM mapping, as there's no indication that this
type of memory was considered).

The cortex_r8_virtual appears to be a stripped down copy
of the old qemu_cortex_r5 codebase, therefore, the duplicated
MPU regions have the same flaws as qemu_cortex_r5 or any
actual ZynqMP-based target for that matter.

Signed-off-by: Immo Birnbaum <mail@birnbaum.immo>