ARM: dts: armada-370-xp: Fixup mdio DT warning

MDIO has a reg property so the unit name should contain an address.
Take the opportunity to use the node label instead of the full name.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
diff --git a/arch/arm/boot/dts/armada-370-xp.dtsi b/arch/arm/boot/dts/armada-370-xp.dtsi
index 5435989..a4f9684 100644
--- a/arch/arm/boot/dts/armada-370-xp.dtsi
+++ b/arch/arm/boot/dts/armada-370-xp.dtsi
@@ -259,7 +259,7 @@
 				status = "disabled";
 			};
 
-			mdio: mdio {
+			mdio: mdio@72004 {
 				#address-cells = <1>;
 				#size-cells = <0>;
 				compatible = "marvell,orion-mdio";