net: phy: add core phylib sfp support

Add core phylib help for supporting SFP sockets on PHYs.  This provides
a mechanism to inform the SFP layer about PHY up/down events, and also
unregister the SFP bus when the PHY is going away.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>