feat(plat/mediatek/mt8186): add SiP service

Add the basic SiP service.

TEST=build pass
BUG=b:202871018

Change-Id: I4dcc7383237bb6c1f2494920cde21197754f6367
diff --git a/plat/mediatek/mt8186/plat_sip_calls.c b/plat/mediatek/mt8186/plat_sip_calls.c
new file mode 100644
index 0000000..716f3d9
--- /dev/null
+++ b/plat/mediatek/mt8186/plat_sip_calls.c
@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2021, MediaTek Inc. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <common/debug.h>
+#include <common/runtime_svc.h>
+
+uintptr_t mediatek_plat_sip_handler(uint32_t smc_fid,
+				u_register_t x1,
+				u_register_t x2,
+				u_register_t x3,
+				u_register_t x4,
+				void *cookie,
+				void *handle,
+				u_register_t flags)
+{
+	switch (smc_fid) {
+	default:
+		ERROR("%s: unhandled SMC (0x%x)\n", __func__, smc_fid);
+		break;
+	}
+
+	SMC_RET1(handle, SMC_UNK);
+}