arch: arm: refactor _region_init(..) function

This commit refactors the _region_init(..) internal MPU driver
function so it gets the region index and a struct arm_mpu_region
as aruments. The latter holds the MPU region configuration that
is to be applied to the region determined by the index argument.

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