arch: arm: Kconfig option for non-overlapping MPU region requirement

This commit introduces a new Kconfig option symbol,
MPU_REQUIRES_NON_OVERLAPPING_REGIONS, to signify the use of an
MPU architecture that requires the active MPU regions be
non-overlapping. This requirement concerns the standard
(unmodified) ARMv8-M MPU implementation. For that, we select
the option in the definition of ARM_MPU, if the CPU architecture
is ARMv8-M (Baseline or Mainline).

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