soc: arm: framework for common fixed MPU region configuration

We move the configuration of the fixed MPU regions for ARM
Cortex-M SoCs in a common place under soc/arm/common/cortex-m,
instead of having this configuration present in each ARM SoC or
SoC Series definition. The rationale behind this is that for all
SoCs the fixed MPU regions configured at SoC definition are only
used for enforcing default Flash and SRAM access policies, and
currently, this is common to all ARM SoCs with MPU support.

We also simplify the Flash and SRAM MPU region definition,
aiming at using a single MPU region index to program each
of them.

We still support the possibility for ARM SoCs to opt-out and,
instead, define their own custom fixed MPU regions at SoC
definition. We do it using a Kconfig option, introduced
explicitly for this purpose.

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