feat(mediatek): add smcc call for MSDC

Some registers of MSDC need to be set in ATF, so we add MSDC drivers.

Change-Id: Idde51a136ad08dbaece0bdaa804b934fca7046b6
diff --git a/plat/mediatek/drivers/msdc/mt_msdc.h b/plat/mediatek/drivers/msdc/mt_msdc.h
new file mode 100644
index 0000000..1c500c2
--- /dev/null
+++ b/plat/mediatek/drivers/msdc/mt_msdc.h
@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2022, MediaTek Inc. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#ifndef MT_MSDC_H
+#define MT_MSDC_H
+
+#include <mt_msdc_priv.h>
+
+uint64_t msdc_smc_dispatcher(uint64_t arg0, uint64_t arg1,
+			     uint64_t arg2, uint64_t arg3);
+
+#endif