dt-bindings: sunxi: Add compatible for OrangePi 3 board

Add new Xunlong Orange Pi 3 board compatible string to the bindings
documentation.
Signed-off-by: Ondrej Jirman <megous@megous.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>

Documentation/devicetree/bindings/arm/sunxi.yaml

@@ -716,6 +716,11 @@ properties:
           - const: xunlong,orangepi-2
           - const: allwinner,sun8i-h3
 
+      - description: Xunlong OrangePi 3
+        items:
+          - const: xunlong,orangepi-3
+          - const: allwinner,sun50i-h6
+
       - description: Xunlong OrangePi Lite
         items:
           - const: xunlong,orangepi-lite