net: mdio: setup of_node for the MDIO device

By mistake, the of_node of the MDIO device was not setup in the patch
linked below. As a consequence, any PHY driver that depends on the
of_node in its probe callback was not be able to successfully finish its
probe on a PHY, thus the Generic PHY driver was used instead.

Fix this by actually setting up the of_node.

Fixes: bc1bee3b ("net: mdiobus: Introduce fwnode_mdiobus_register_phy()")
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>