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