aarch64: mmu: Avoid creating a new table when not needed

In the current MMU code a new table is created when mapping a memory
region that is overlapping with a block already mapped. The problem is
that the new table is created also when the new and old mappings have
the same attributes.

To avoid using a new table when not needed the attributes of the two
mappings are compared before creating the new table.

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