fix(mediatek): add mt8188 plat build in CI

Signed-off-by: Bo-Chen Chen <rex-bc.chen@mediatek.com>
Change-Id: Ib2f4dbd15ca7a49e4a1d05c74fb000cbab825905
diff --git a/group/tf-l1-build-plat/mt8188-default:nil b/group/tf-l1-build-plat/mt8188-default:nil
new file mode 100644
index 0000000..35f8713
--- /dev/null
+++ b/group/tf-l1-build-plat/mt8188-default:nil
@@ -0,0 +1,5 @@
+#
+# Copyright (c) 2022 Arm Limited. All rights reserved.
+#
+# SPDX-License-Identifier: BSD-3-Clause
+#