xlat lib: Don't set mmap_attr_t enum to be -1

-1 is not a defined mmap_attr_t type. Instead of using invalid enum
types, we can either choose to define a INVALID type OR handle the
condition specifically.

Since the usage of mmap_region_attr is limited, it is easier to just
handle the error condition specifically and return 0 or -1 depending
on success or fail.

Fixes: ARM-Software/tf-issues#473
Fixes: 28fa2e9e ("xlat lib: Use mmap_attr_t type consistently")

Signed-off-by: Nishanth Menon <nm@ti.com>