dt-bindings: display: sun4i-drm: Add H6 display engine compatibles

This commit adds compatibles used in H6 display pipeline, namely for
display engine, mixer and TV TCON.

H6 display engine is somewhat similar to R40, just less TCONs and
mixer support more features.

Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181104182705.18047-8-jernej.skrabec@siol.net