Specify integration as the default branch for git-review

Submitting a change for review using 'git review' will now automatically use
the special refs/for/integration ref (instead of targeting the master
branch).

Change-Id: Idef58b20c492bf5ab06599f4cd4a5e5b75837066
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

.gitreview

@@ -2,3 +2,4 @@
 host=review.trustedfirmware.org
 port=29418
 project=TF-A/trusted-firmware-a
+defaultbranch=integration