Define fwnode_mdio_find_device() to get a pointer to the
mdio_device from fwnode passed to the function.

Refactor of_mdio_find_device() to use fwnode_mdio_find_device().
Signed-off-by: NCalvin Johnson <calvin.johnson@oss.nxp.com>
Signed-off-by: NIoana Ciornei <ioana.ciornei@nxp.com>
Acked-by: NGrant Likely <grant.likely@arm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>