rockchip: Add support for Raxda Rock 2

This board includes an RK3288 SoC on a SOM. It can be mounted on a
base-board which provides a wide range of peripherals.

So far this is verified to boot to a prompt from a microSD card. The serial
console works as well as HDMI.

Thanks to Tom Cubie for sending me a board.

Signed-off-by: Simon Glass <sjg@chromium.org>