Switch TF-M branch from "master" to "main"

TF-M development branch is going to be switched from "master" to "main".
Update CI settings to enable tests on "main" branch.

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I3cb37e0f777d117b4ed2081125cfcebd59ed7472
diff --git a/tf-m-build-docs-nightly.yaml b/tf-m-build-docs-nightly.yaml
index e426ed6..5c005eb 100644
--- a/tf-m-build-docs-nightly.yaml
+++ b/tf-m-build-docs-nightly.yaml
@@ -21,7 +21,7 @@
     parameters:
       - string:
           name: 'GERRIT_REFSPEC'
-          default: 'refs/heads/master'
+          default: 'refs/heads/main'
           description: |
             Specify which TF-M version to use
             refs/heads/<branch_name> : Pick up latest HEAD on branch
@@ -29,7 +29,7 @@
             full commit hash         : Pick up a specific commit
       - string:
           name: 'GERRIT_BRANCH'
-          default: 'master'
+          default: 'main'
           description: |
             TF-M branch.
       - string: