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