dt-bindings: display: sun4i-drm: Add R40 display engine compatible

R40 has pretty unique display pipeline. It supports two outputs at the
same time.

Possible outputs:
- 1x HDMI,
- 2x TV output
- 1x VGA,
- 1x MIPI DSI and
- 2x LCD outputs

That is the biggest number of possible outputs from all Allwinner SoC.
Because of that, add new compatible for it.
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/20180710203511.18454-2-jernej.skrabec@siol.net

Documentation/devicetree/bindings/display/sunxi/sun4i-drm.txt

@@ -399,6 +399,7 @@ Required properties:
     * allwinner,sun8i-a33-display-engine
     * allwinner,sun8i-a83t-display-engine
     * allwinner,sun8i-h3-display-engine
+    * allwinner,sun8i-r40-display-engine
     * allwinner,sun8i-v3s-display-engine
     * allwinner,sun9i-a80-display-engine