mdio_bus: Generalise of_mdiobus_link_phydev()

This function should work with any sort of MDIO device which can be
probed on the bus, not just PHY devices. So generalise it.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>