Restore some defines in xlat_tables.h

Commit e8719552 removed some definitions related to translation
tables from the xlat_tables.h header file, based on the assumption
that they weren't used by any platform. These are actually used by
some partners so this patch restores them.

Fixes ARM-software/tf-issues#425

Change-Id: Idafa5f00bb0bd9c2847b5ae6541cf8db93c7b89a