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>