dt-bindings: display: sun4i-drm: Add R40 mixer compatibles

R40 DE2 mixers are similar to those found in A83T, except it needs
different clock settings.

Add a compatibles for them.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180625120304.7543-14-jernej.skrabec@siol.net