ARM: mvebu: add alias for mdio on Armada 38x

This allows us to reference it later.
Signed-off-by: Imre Kaloz <kaloz@openwrt.org>

arch/arm/boot/dts/armada-38x.dtsi

@@ -495,7 +495,7 @@ eth0: ethernet@70000 {
 				status = "disabled";
 			};
 
-			mdio@72004 {
+			mdio: mdio@72004 {
 				#address-cells = <1>;
 				#size-cells = <0>;
 				compatible = "marvell,orion-mdio";