drm/sun4i: Add support for A83T second DE2 mixer

It supports 1 VI and 1 UI plane and HW scaling on both planes.

Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
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/20180214200906.31509-10-jernej.skrabec@siol.net