feat(mt8189): initialize platform for MT8189

- Add basic platform setup.
- Add MT8189 documentation at docs/plat/.
- Add generic CPU helper functions.
- Add basic register address.

Change-Id: Id59ae9265983defb46e27befabfd5c30b2b4a5a6
diff --git a/docs/plat/index.rst b/docs/plat/index.rst
index 0b53d1d..0ad9181 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -22,6 +22,7 @@
    mt8183
    mt8186
    mt8188
+   mt8189
    mt8192
    mt8195
    mt8196