drm/sun4i: Add compatible strings for A31/A31s display pipelines

The A31's display pipeline has 2 frontends, 2 backends, and 2 TCONs. It
also has new display enhancement blocks, such as the DRC (Dynamic Range
Controller), the DEU (Display Enhancement Unit), and the CMU (Color
Management Unit). It supports HDMI, MIPI DSI, and 2 LCD/LVDS channels.

The A31s display pipeline is almost the same, just without MIPI DSI.
Only the TCON seems to be different, due to the missing mux for MIPI
DSI.

Add compatible strings for both of them.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

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

@@ -91,6 +91,7 @@ system.
 Required properties:
   - compatible: value must be one of:
     * allwinner,sun5i-a13-display-backend
+    * allwinner,sun6i-a31-display-backend
     * allwinner,sun8i-a33-display-backend
   - reg: base address and size of the memory-mapped region.
   - clocks: phandles to the clocks feeding the frontend and backend
@@ -121,6 +122,7 @@ deinterlacing and color space conversion.
 Required properties:
   - compatible: value must be one of:
     * allwinner,sun5i-a13-display-frontend
+    * allwinner,sun6i-a31-display-frontend
     * allwinner,sun8i-a33-display-frontend
   - reg: base address and size of the memory-mapped region.
   - interrupts: interrupt associated to this IP
@@ -146,6 +148,8 @@ extra node.
 Required properties:
   - compatible: value must be one of:
     * allwinner,sun5i-a13-display-engine
+    * allwinner,sun6i-a31-display-engine
+    * allwinner,sun6i-a31s-display-engine
     * allwinner,sun8i-a33-display-engine
 
   - allwinner,pipelines: list of phandle to the display engine