net: davinci_emac: match the mdio device against its compatible if possible

Device tree based systems without of_dev_auxdata will have the mdio
device named differently than "davinci_mdio(.0)". In this case use the
device's parent's compatible string for matching

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>