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-builder/builders.sh b/tf-a-builder/builders.sh
index be8426d..a591bf8 100755
--- a/tf-a-builder/builders.sh
+++ b/tf-a-builder/builders.sh
@@ -122,6 +122,7 @@
 export tf_root="${tf_root:-${WORKSPACE}/trusted-firmware-a}"
 export tftf_root="${tftf_root:-${WORKSPACE}/tf-a-tests}"
 export spm_root="${spm_root:-${WORKSPACE}/spm}"
+export rmm_root="${rmm_root:-${WORKSPACE}/tf-rmm}"
 
 # We'd need to see the terminals during development runs, so no need for
 # automation.