scp: Update repo references to Gitlab

SCP-firmware repository has moved from GitHub to Gitlab (the
Github instance is not accepting pull requests any longer).

Update URLs in the contributing and user_guide to point
to the active Gitlab repository.
Simplify descriptions at the same time for submitting changes.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: I1417b4172c54ef0b4d40644416bd9e6995ee3079