xlat lib: Use mmap_attr_t type consistently

This patch modifies both versions of the translation table library
to use the mmap_attr_t type consistently wherever it is manipulating
MT_* attributes variables. It used to use mmap_attr_t or plain integer
types interchangeably, which compiles fine because an enumeration type
can be silently converted to an integer, but which is semantically
incorrect.

This patch removes this assumption by using the abstract type
'mmap_attr_t' all the time.

Change-Id: Id1f099025d2cb962b275bb7e39ad2c4dbb4e366c
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>