arm: mvebu: enable Ethernet controllers on OpenBlocks AX3-4 platform

The PlatHome OpenBlocks AX3-4 platform has 4 Ethernet ports, connected
to a single quad-port PHY through SGMII.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

arch/arm/boot/dts/armada-xp-openblocks-ax3-4.dts

@@ -65,5 +65,48 @@ green_led {
 				linux,default-trigger = "heartbeat";
 			};
 		};
+
+		mdio {
+			phy0: ethernet-phy@0 {
+				reg = <0>;
+			};
+
+			phy1: ethernet-phy@1 {
+				reg = <1>;
+			};
+
+			phy2: ethernet-phy@2 {
+				reg = <2>;
+			};
+
+			phy3: ethernet-phy@3 {
+				reg = <3>;
+			};
+		};
+
+		ethernet@d0070000 {
+			clock-frequency = <250000000>;
+			status = "okay";
+			phy = <&phy0>;
+			phy-mode = "sgmii";
+		};
+		ethernet@d0074000 {
+			clock-frequency = <250000000>;
+			status = "okay";
+			phy = <&phy1>;
+			phy-mode = "sgmii";
+		};
+		ethernet@d0030000 {
+			clock-frequency = <250000000>;
+			status = "okay";
+			phy = <&phy2>;
+			phy-mode = "sgmii";
+		};
+		ethernet@d0034000 {
+			clock-frequency = <250000000>;
+			status = "okay";
+			phy = <&phy3>;
+			phy-mode = "sgmii";
+		};
 	};
 };