net: phy: phylink: Allow setting a custom link state callback

phylink_get_fixed_state() currently consults an optional "link_gpio"
GPIO descriptor, expand this mechanism to allow specifying a custom
callback. This is necessary to support out of band link notifcation
(e.g: from an interrupt within a MMIO register).

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