of: of_mdio: export symbol of_mdiobus_link_phydev

Make of_mdiobus_link_phydev externally available.
This fixes CONFIG_OF_MDIO=m.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 86f6cf41 ("net: of_mdio: add of_mdiobus_link_phydev()")
Signed-off-by: David S. Miller <davem@davemloft.net>