hwmon: (gpio-fan) Move DT bindings to the right place

This moves the GPIO fan bindings to the hwmon bindings. The GPIO
fan is a hwmon class hardware, not related to GPIO other than being
a consumer of GPIOs.

Cc: devicetree@vger.kernel.org
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>