mmu: get virtual alignment from physical address

On ARM64 platforms, when mapping multiple memory zones with size
not multiple of a L2 block size (2MiB), all the following mappings
will probably use L3 tables.

And a huge mapping will consume all possible L3 tables.

In order to reduce usage of L3 tables, this introduces a new
arch_virt_region_align() optional architecture specific
call to eventually return a more optimal virtual address
alignment than the default MMU_PAGE_SIZE.

This alignment is used in virt_region_alloc() by:
- requesting more pages in virt_region_bitmap to make sure we request
  up to the possible aligned virtual address
- freeing the supplementary pages used for alignment

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