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

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

@@ -376,6 +376,8 @@ Required properties:
     * allwinner,sun8i-a83t-de2-mixer-0
     * allwinner,sun8i-a83t-de2-mixer-1
     * allwinner,sun8i-h3-de2-mixer-0
+    * allwinner,sun8i-r40-de2-mixer-0
+    * allwinner,sun8i-r40-de2-mixer-1
     * allwinner,sun8i-v3s-de2-mixer
     * allwinner,sun50i-a64-de2-mixer-0
     * allwinner,sun50i-a64-de2-mixer-1