arch: arm: refactor _region_init(..) function
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 <5921cc8bab7e1d4329f52fd8f6268f9692e3de80@nordicsemi.no>

zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,nashif/zephyr,finikorg/zephyr,kraj/zephyr,galak/zephyr,nashif/zephyr,ldts/zephyr,explora26/zephyr,Vudentz/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,finikorg/zephyr,nashif/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,galak/zephyr,kraj/zephyr,Vudentz/zephyr,explora26/zephyr,nashif/zephyr,punitvara/zephyr,nashif/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,punitvara/zephyr,Vudentz/zephyr,punitvara/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,punitvara/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,Vudentz/zephyr,explora26/zephyr,ldts/zephyr