net: mdio: of: export part of of_mdiobus_register_phy()

This function will be needed in tja11xx driver for secondary PHY
support.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>