feat(mt8188): initialize platform for MediaTek MT8188

- Add basic platform setup.
- Add MT8188 documentation at docs/plat/.
- Add generic CPU helper functions.
- Add basic register address.
- Add mtk_pm.c in lib/pm

TEST=build pass
BUG=b:236331724

Change-Id: I5f8617c42ffba2c9d3a16f3980cb75fda5624031
diff --git a/plat/mediatek/lib/pm/mtk_pm.c b/plat/mediatek/lib/pm/mtk_pm.c
new file mode 100644
index 0000000..632a1e7
--- /dev/null
+++ b/plat/mediatek/lib/pm/mtk_pm.c
@@ -0,0 +1,18 @@
+/*
+ * Copyright (c) 2022, MediaTek Inc. All rights reserved.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <lib/psci/psci.h>
+
+static const plat_psci_ops_t plat_psci_ops = {
+};
+
+int plat_setup_psci_ops(uintptr_t sec_entrypoint,
+			const plat_psci_ops_t **psci_ops)
+{
+	*psci_ops = &plat_psci_ops;
+
+	return 0;
+}