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 <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Maxime Ripard <[e-mail]>

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

···
 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
···
 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
···
 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

drivers/gpu/drm/sun4i/sun4i_backend.c

···
 
 static const struct of_device_id sun4i_backend_of_table[] = {
 	{ .compatible = "allwinner,sun5i-a13-display-backend" },
+	{ .compatible = "allwinner,sun6i-a31-display-backend" },
 	{ .compatible = "allwinner,sun8i-a33-display-backend" },
 	{ }
 };

drivers/gpu/drm/sun4i/sun4i_drv.c

···
 static bool sun4i_drv_node_is_frontend(struct device_node *node)
 {
 	return of_device_is_compatible(node, "allwinner,sun5i-a13-display-frontend") ||
+		of_device_is_compatible(node, "allwinner,sun6i-a31-display-frontend") ||
 		of_device_is_compatible(node, "allwinner,sun8i-a33-display-frontend");
 }
 
···
 
 static const struct of_device_id sun4i_drv_of_table[] = {
 	{ .compatible = "allwinner,sun5i-a13-display-engine" },
+	{ .compatible = "allwinner,sun6i-a31-display-engine" },
+	{ .compatible = "allwinner,sun6i-a31s-display-engine" },
 	{ .compatible = "allwinner,sun8i-a33-display-engine" },
 	{ }
 };