arm64: dts: allwinner: a64: Add pinmux for RGB666 LCD

Allwinner A64's TCON0 can output RGB666 LCD signal.

Add its pinmux.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Torsten Duwe <duwe@suse.de>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>