ipq40xx: edma: convert to of_mdio_find_bus()

ipq40xx: edma: convert to of_mdio_find_bus()

With the reworked MDIO driver, EDMA will fail to get the MII BUS as it
used the MII BUS stored inside the MDIO structure private data.

This obviously does not work with the modernized driver, so lets switch
to using a purpose build of_mdio_find_bus() which will return the MII
BUS and only requires the MDIO node to be passed.
This is easy as we already have the node parsed.

Also, since we now require OF_MDIO add that as dependency.

Signed-off-by: Robert Marko <robert.marko@sartura.hr>