ARM: dts: BCM5301X: Specify MDIO bus in the DT

Northstar devices have MDIO bus that may contain various PHYs attached.
A common example is USB 3.0 PHY (that doesn't have an MDIO driver yet).
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

arch/arm/boot/dts/bcm5301x.dtsi

@@ -320,6 +320,14 @@ gmac3: ethernet@27000 {
 		};
 	};
 
+	mdio: mdio@18003000 {
+		compatible = "brcm,iproc-mdio";
+		reg = <0x18003000 0x8>;
+		#size-cells = <1>;
+		#address-cells = <0>;
+		status = "disabled";
+	};
+
 	i2c0: i2c@18009000 {
 		compatible = "brcm,iproc-i2c";
 		reg = <0x18009000 0x50>;