ARM: sun8i: v3s: add device nodes for DE2 display pipeline

Allwinner V3s SoC features a "Display Engine 2.0" with only one mixer
and only one TCON connected to this mixer, which have RGB LCD output.

Add device nodes for this display pipeline.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>