aarch64: mmu: Move xlat tables to one single array

In the current code the base xlat table is a standalone array. This is
done because we know at compile time the size of this table so we can
allocate the correct size and save a bit of memory. All the other xlat
tables are statically allocated in a different array with full size.

With this patch we move all the page tables in one single array,
including the base table. This is probably going to waste a bit of space
but it makes easier to:

- have all the page tables mapped in one single contiguous memory region
  instead of having to take care of two different arrays in two
  different locations
- duplicate the page tables more quickly if we need to
- use a pre-allocated space to host the page tables
- use a pre-computed set of page tables saved in a contiguous memory
  region

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