refactor(xlat): add several refactorizations to the xlat library

This patch includes a number of refactorizations of the xlat library in
order to facilitate the implementation of unittests for the library.
Amongst the improvements there are:

    * Macros to allocate and initialize the translation tables as well
      as the translation context structures have been removed and
      replaced with library APIs.
    * Some of the library APIs have been collapsed, reducing the size
      of the overall interface and presenting a more compact API to
      the consumer.
    * The distinction between the translation base tables and the
      intermediate tables have been removed, presenting now a single
      array of tables for the whole translation context.

The patch also adapts the rest of RMM to use the new library API.

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: I2fb486c0c5bc005446b09e3fef5e7de9bf0efda0