arm64: dts: allwinner: add snps,dwmac-mdio compatible to emac/mdio

stmmac bindings docs said that its mdio node must have
compatible = "snps,dwmac-mdio";
Since dwmac-sun8i does not have any good reasons to not doing it, all
their MDIO node must have it.
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

arch/arm64/boot/dts/allwinner/sun50i-a64.dtsi

@@ -520,6 +520,7 @@ emac: ethernet@1c30000 {
 			#size-cells = <0>;
 
 			mdio: mdio {
+				compatible = "snps,dwmac-mdio";
 				#address-cells = <1>;
 				#size-cells = <0>;
 			};