net: phylink: support for link gpio interrupt

Add support for using GPIO interrupts with a fixed-link GPIO rather than
polling the GPIO every second and invoking the phylink resolution.  This
avoids unnecessary calls to mac_config().

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>