docs(porting): remove pull request terminology

The pull request terminology dates back from when TF-A repository was
hosted on Github. Use a terminology that is more suited to Gerrit
workflow.

Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>
Change-Id: Ieecf47617ca1cdb76b9c4a83f63ba3c402b9e975
1 file changed