arch: arm: refactor _get_region_attr_by_type() function

This commit refactors the _get_region_attr_by_type(.) function,
so it does the following:
 - it takes an arm_mpu_region_attr_t object pointer and use it
   to fill-in the derived region attributes.
 - it performs only the distinguishing between the different MPU
    region types, hiding the actual MPU-specific implementation
	in internal functions.
 - it takes an additional argument (the region base address),
   which allows the function to have a common signature with the
   ARMv8-M version of it (where the attribute derivation needs
   the base address).
 - it returs 0 or succes and a negative non-zero integer
   to indicate an error.

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