arch: arm: adapt region_init(.) to use arm_mpu_region_attr structure

This commit adapts the _region_init(.) function so it can use
the rasr field of the arm_mpu_region_attr structure, when it
configures the RASR register.

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