arch: arm: type definition for arm mpu attribute container

This commit introduces a type definition for the ARM MPU
region attribute container. This allows to abstract the type
of the attribute container and make the code extendible for
ARMv8-M, where the size and structure of the attribute
container will be different.

Therefore, we can, now, move the definition of the region
data structure in the common arm_mpu.h header.

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