ARM: dts: armada-375: Fixup mdio DT warning

MDIO has a reg property so the unit name should contain an address.
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>

arch/arm/boot/dts/armada-375.dtsi

@@ -203,7 +203,7 @@ gic: interrupt-controller@d000 {
 				      <0xc100 0x100>;
 			};
 
-			mdio: mdio {
+			mdio: mdio@c0054 {
 				#address-cells = <1>;
 				#size-cells = <0>;
 				compatible = "marvell,orion-mdio";