dt-bindings: net: rockchip-dwmac: add rk3568 compatible string

Add compatible string for RK3568 gmac, and constrain it to
be compatible with Synopsys dwmac 4.20a.
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/rockchip-dwmac.yaml

@@ -24,6 +24,7 @@ select:
           - rockchip,rk3366-gmac
           - rockchip,rk3368-gmac
           - rockchip,rk3399-gmac
+          - rockchip,rk3568-gmac
           - rockchip,rv1108-gmac
   required:
     - compatible
@@ -33,7 +34,8 @@ allOf:
 
 properties:
   compatible:
-    items:
+    oneOf:
+      - items:
           - enum:
               - rockchip,px30-gmac
               - rockchip,rk3128-gmac
@@ -45,6 +47,10 @@ properties:
               - rockchip,rk3368-gmac
               - rockchip,rk3399-gmac
               - rockchip,rv1108-gmac
+      - items:
+          - enum:
+              - rockchip,rk3568-gmac
+          - const: snps,dwmac-4.20a
 
   clocks:
     minItems: 5