clone.sh: Switch default dependency branch to "main"

TF-M repos are going to switch development branch from "master" to
"main". So update the default dependency branch in CI to "main".

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: If64a9e90bcbde092b2d6c5f0761f2fe9088d80bc
diff --git a/clone.sh b/clone.sh
index fbb5ec2..bf8b631 100755
--- a/clone.sh
+++ b/clone.sh
@@ -53,7 +53,7 @@
     VERSION="$(grep "set(${DEPENDENCY_NAME}" ${CONFIG_FILE_PATH} | cut -d\" -f2)"
 
     if [ -z "${VERSION}" ]; then
-        VERSION="refs/heads/master"
+        VERSION="refs/heads/main"
     fi
 
     echo "${VERSION}"