arch: arm: mpu: protect RNR when reading RBAR, RASR in ARMv7-M driver

We lock IRQs around writing to RNR and immediate reading of RBAR
RASR in ARMv7-M MPU driver. We do this for the functions invoked
directly or undirectly by arch_buffer_validate(). This locking
guarantees that
- arch_buffer_validate() calls by ISRs may safely preempt each
  other
- arch_buffer_validate() calls by threads may safely preempt
  each other (i.e via context switch -out and -in again).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>