arch: arm: abstract MPU attribute generation in inline function

This commit abstracts the attribution generation for an MPU
region, determined based on a supplied configuration and the
given size, from the actual implementation, which is placed
in a separate inline function. This allows to have multiple
implementations of attribution generation for different ARM
MPU drivers.

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