ARM: dts: sun6i: Add A31 LCD0 RGB888 pins

The LCD0 controller on the A31 can do RGB output up to 8 bits per
channel. Add the pins for RGB888 output.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>