phy: fixed_phy: Add gpio to determine link up/down.

An SFP module may have a link up/down status pin which can be
connection to a GPIO line of the host. Add support for reading such an
GPIO in the fixed_phy driver.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>