feat(mt8186): initialize platform for MediaTek MT8186

- Add basic platform setup.
- Add MT8186 documentation at docs/plat/.
- Add generic CPU helper functions.
- Add basic register address.

TEST=build pass
BUG=b:202871018

Change-Id: Id3e2f46a8c3ab2f3e29137e508d4c671e8f4aad5
diff --git a/docs/plat/index.rst b/docs/plat/index.rst
index 5848005..eadd946 100644
--- a/docs/plat/index.rst
+++ b/docs/plat/index.rst
@@ -20,6 +20,7 @@
    intel-stratix10
    marvell/index
    mt8183
+   mt8186
    mt8192
    mt8195
    nvidia-tegra