drm/sun4i: Add DT binding for Detail Enhancement Unit in Allwinner A80 SoC

The display pipeline on the A80 SoC has what is called the Detail
Enhancement Unit, or DEU for short, block in between the display
frontend and backend. This unit can sharpen images in both luma
and chroma channels. It seems to also do colorspace conversion.

This patch adds the device tree binding for this hardware block.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180315114136.24747-3-wens@csie.org