xlat: Use MAP_REGION macro as compatibility layer

Use the MAP_REGION to build the mmap_region_t argument in wrappers like
mmap_add_region(). Evolution of the mmap_region_t might require adding
new members with a non-zero default value. Users of MAP_REGION are
protected against such evolution. This commit also protects users of
mmap_add_region() and mmap_add_dynamic_region() functions against these
evolutions.

Also make the MAP_REGION macro implementation more explicit and make it
a mmap_region_t compound literal to make it useable as a function
parameter on its own and to prevent using it in initialization of
variables of different type.

Change-Id: I7bfc4689f6dd4dd23c895b65f628d8ee991fc161
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>