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.

Since these compatible is automatically registered, dwmac-sun8i compatible
does not need to be in need_mdio_ids.
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>