aarch64: mmu: Makes memory mapping functions more generic

The ARM64 MMU code used to create the page tables is strictly tied to
the custom arm_mmu_region struct. To be able to hook up this code to the
Zephyr MMU APIs we need to make it more generic.

This patch makes the mapping function more generic and creates a new
helper function add_arm_mmu_region() to map the regions defined by the
old arm_mmu_region structs using this new generic function.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>