drm/sun4i: Add support for R40 mixers

Both mixers have similar capabilities as others SoCs with DE2.

First mixer has 1 VI and 3 UI planes and supports HW scaling on all
planes.

Second mixer has 1 VI and 1 UI planes and also supports HW scaling on
all planes.

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/20180625120304.7543-15-jernej.skrabec@siol.net