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Instead of having one big file with all the code, it's better to have
a few smaller files that are more manageable:

- xlat_tables_core.c: Code related to the core functionality of the
  library (map and unmap regions, initialize xlat context).
- xlat_tables_context.c: Instantiation of the active image context
  as well as APIs to manipulate it.
- xlat_tables_utils.c: Helper code that isn't part of the core
  functionality (change attributes, debug print messages).

Change-Id: I3ea956fc1afd7473c0bb5e7c6aab3b2e5d88c711
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>