of_mdio: add helper to deregister fixed-link PHYs

Add helper to deregister fixed-link PHYs registered using
of_phy_register_fixed_link().

Convert the two drivers that care to deregister their fixed-link PHYs to
use the new helper, but note that most drivers currently fail to do so.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>