ath79: mdio: add address-cells and size-cells defaults

Eliminates warnings about relying on defaults from the device tree
compiler.

Signed-off-by: Karl Palsson <karlp@etactica.com>

target/linux/ath79/dts/ath79.dtsi

@@ -51,6 +51,8 @@
 
 			mdio0: mdio-bus {
 				status = "disabled";
+				#address-cells = <1>;
+				#size-cells = <0>;
 
 				regmap = <&eth0>;
 
@@ -70,6 +72,8 @@
 
 			mdio1: mdio-bus {
 				status = "disabled";
+				#address-cells = <1>;
+				#size-cells = <0>;
 
 				regmap = <&eth1>;