Core(M): Aligned PSPLIM and MSPLIM access functions among compilers and device variants.

Core(M): Aligned PSPLIM and MSPLIM access functions among compilers and device variants.
Non-secure PSPLIM and MSPLIM are RAZ/WI if Main Extensions are not implemented.
According to an advice from ATEG team in SDDKW-43532 we shall not rely on hardware RAZ/WI behaviour.
Thus the access functions now mimic RAZ/WI behaviour if the registers are not available.
- Fixed/adopted implementation for ArmClang, GCC and IAR.
- Enhanced CoreValidation tests.
- Enhanced doxygen documentation.