Mandate 'Signed-off-by' line in commit messages

This patch updates the configuration file for the checkpatch.pl
script to check for the presence of a 'Signed-off-by' line in the
commit message. This is now required by TF contribution process.

Change-Id: I8bcd6d39a5dcd85547244d60c10a7cd2148c404f
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

.checkpatch.conf

@@ -36,9 +36,6 @@
 # This is not Linux so don't expect a Linux tree!
 --no-tree
 
-# 'Signed-off-by' lines in commit messages are not mandated for TF.
---no-signoff
-
 # This clarifes the lines indications in the report.
 #
 # E.g.: