arch: arm: mpu: implement arm_core_mpu_get_max_available_dyn_regions

This commit adds the implementation of internal ARM MPU
function, arm_core_mpu_get_max_available_dyn_regions(),
required by _arch_mem_domain_max_partitions_get(),
according to the new architecture. The function returns
the number of the available partitions for memory domains
based on the total amount of HW regions and the
actual number of statically configured MPU regions.

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