Add support for cloning the TF-RMM repository

Enhance the tf-a-ci-gateway job to include the capability to clone the TF-RMM repository.
Updated the clone script accordingly to handle this new addition.

Change-Id: I3dae9befbd686a8252b8f3bd7dd3e2b500dd022b
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/tf-a-ci-gateway.yaml b/tf-a-ci-gateway.yaml
index faa1134..15b14f0 100644
--- a/tf-a-ci-gateway.yaml
+++ b/tf-a-ci-gateway.yaml
@@ -66,6 +66,11 @@
         description: |
           tf-a-ci-scripts refspec to use. The master branch is used by default.
     - string:
+        name: RMM_REFSPEC
+        default: '+refs/heads/main:refs/remotes/origin/main'
+        description: |
+          tf-rmm refspec to use. The main branch is used by default.
+    - string:
         name: JOBS_REFSPEC
         default: 'refs/heads/master'
         description: |