phy: add phy_device_remove()

Add a phy_device_remove() function to complement phy_device_register(),
which undoes the effects of phy_device_register() by removing the phy
device from visibility, but not freeing it.

This allows these details to be moved out of the mdio bus code into
the phy code where this action belongs.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>