aarch64: mmu: cleanups and fixes

Major changes:

- move related functions together
- optimize add_map() not to walk the page tables *twice* on
  every loop
- properly handle leftover size when a range is already mapped
- don't overwrite existing mappings by default
- return an error when the mapping fails

and make the code clearer overall.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>