net: axienet: Fix MDIO bus parent node detection

This driver was previously using the parent node of the specified PHY
node as the device node to register the MDIO bus on. Andrew Lunn
pointed out this is wrong as the PHY node is potentially not even
underneath the MDIO bus for the current device instance. Find the MDIO
node explicitly by looking it up by name under the controller's device
node instead.

This could potentially break existing device trees if they don't use
"mdio" as the name for the MDIO bus, but I did not find any with various
searches and Xilinx's examples all use mdio as the name so it seems like
this should be relatively safe.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>