rockchip: Add basic support for firefly-rk3288

The Firefly RK3288 is a suitable target board for initial mainline Rockchip
support. It includes a good set of peripherals, a recent SoC and it is
readily available.

This adds only some basic files required to allow the baord to display a
serial message in SPL and hang.

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