ARM: sun6i: Add WITS Colombus A31 evaluation kit support

This platform from WITS is the evaluation board for the Allwinner A31.
It features a quad-Cortex A7, 2048MB of RAM, NAND, USB, MMC, several
UART, HDMI, a 2048 x 1536 10" screen, powered by a PowerVR, etc.

Of course, most of these peripherals aren't supported yet, but support
for those will come eventually.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>