xlat: fix mm copy when adding a region

mmap_add_region_ctx and mmap_add_dynamic_region_ctx are clearing members
that they are not aware of by copying each member one by one. Replace
this by structure assignment.

Change-Id: I7c70cb408c8a8eb551402a5d8d956c1fb7f32b55
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>