arch: arm: refactor _get_region_attr_by_conf(.) function

This commit refactors the _get_region_attr_by_conf(.) function,
so it does the following:
 - it is renamed to _get_ram_region_attr_by_conf(), to reflect
   that it is only used to configure RAM MPU regions.
 - it gets an arm_mpu_region_attr_t object structure pointer
   to fill-in with the derived region attributes.
 - 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).

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