In ARMv8-M MPU it is not possible to have the following access
permissions: Privileged RW / Unprivileged RO. So we define
K_MEM_PARTITION_IS_WRITABLE macro separately for v8M and v7M MPU
architectures (in the separate include files).
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>