drm/sun4i: Add support for A64 mixers

Mixers in Allwinner have similar capabilities as others SoCs with DE2.

Add support for them.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
[Icenowy: Add mixer1]
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Reviewed-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180904044053.15425-5-icenowy@aosc.io