xlat: Remove references to the Trusted Firmware

This library can be used in other projects. All comments that talk about
the Trusted Firmware should be talking about the library itself.

Change-Id: I3b98d42f7132be72c1f8a4900acfaa78dbd2daa2
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>