fix(arm): fix comments referencing SGI platform

Currently, both N1SDP and TC platforms have references to "SGI" within
a comment in their respective source files. Fix this by replacing it
with the appropriate platform name.

Signed-off-by: Rohit Mathew <Rohit.Mathew@arm.com>
Change-Id: I523c3ef0bfa76d35819a8f776a08820ffd197c44