arm64: mmu: implement arch_virt_region_align()

Add the arm64 MMU arch_virt_region_align() implementation used
to return a possible virtual addres alignment in order to
optimize the MMU table layout and possibly avoid using L3 tables
and use some L1 & L3 blocks instead for most of the mapping.

Suggested-by: Nicolas Pitre <npitre@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>