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
13 files changed