hwmon: (gpio-fan) allow to use alarm support alone from DT

On some boards, such as the LaCie 2Big Network v2 or 2Big NAS (based on
Marvell Kirkwood SoCs), an I2C fan controller is used but the alarm
signal is wired to a separate GPIO. Unfortunately, the gpio-fan driver
can't be used to handle GPIO alarm alone from DT: an error is returned
if the "gpios" DT property is missing.

This patch allows to use the gpio-fan driver even if the "alarm-gpios"
DT property is defined alone.

Signed-off-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>