This commit updates the mem_domain_apis_test sample and the
mem_protect test, so they can compile and execute in ARMv8-M
platforms, which do not support the P_RW_U_RO access permissions
combination (privileged read/write, unprivileged read-only). The
modification consists of, simply, selecting a different access
permission (P_RO_U_RO) when building for ARMv8-M MPUs with the
unmodified ARM MPU architecture.
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>