ARM: dts: sun9i: Add pinmux settings for LCD0 RGB888 output.

The A80 supports RGB888 with H/V sync from LCD0. Add a pinmux setting
for the needed pins.

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