[mt8156s_ref] Initial support for MT8167S reference board

Zircon boots into serial console shell on MT8167S with this change.
The Zircon image is packaged to work with Android Verified Boot.
A stub board driver for the board is included in this CL.

Still to do:

- Implement interrupt support in the UART driver instead of spinning and polling.
This will require pinmuxing the interrupt to level triggered active low,
since that configuration cannot be done in the gic.

- Remove code in the boot shim that disables the watchdog timer when we have a proper
watchdog driver in userspace.

TEST: Board boots into a shell, board driver loads and "runtests" passes.

Change-Id: I2b4fa280c40b7deab061033ffcbee04f8584c407