Add JOBS_REFSPEC parameter to jobs

Add JOBS_RESPEC parameter to jobs, and set default value
to master branch.

Change-Id: I779f163dc494df07514f4f0b5034c6113afda109
Signed-off-by: Saheer Babu <saheer.babu@arm.com>
(cherry picked from commit 1e3f00ec2b70b98a945e7a4104bb0eca37dcac0b)
diff --git a/tf-a-lts2.8-main.yaml b/tf-a-lts2.8-main.yaml
index 52722ac..a239fa3 100644
--- a/tf-a-lts2.8-main.yaml
+++ b/tf-a-lts2.8-main.yaml
@@ -41,6 +41,11 @@
           name: CI_REFSPEC
           default: '+refs/heads/lts-v2.8:refs/remotes/origin/lts-v2.8'
       - string:
+          name: JOBS_REFSPEC
+          default: 'refs/heads/master'
+          description: |
+            tf-a-job-configs refspec to use. The master branch is used by default.
+      - string:
           name: MBEDTLS_URL
           default: 'https://github.com/Mbed-TLS/mbedtls/archive/mbedtls-3.6.2.tar.gz'
       - string: