hwmon: Add devicetree bindings to gpio-fan

Allow a gpio-fan to be defined in devicetree, see binding documentation
for details.

Signed-off-by: Jamie Lentin <jm@lentin.co.uk>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>