hwmon: (gpio-fan) Add a shutdown handler to poweroff the fans

Poweroff the fans when shutting down the system. Else,
echo '1' > /sys/class/hwmon/hwmon0/fan1_target; poweroff leaves the
fan running if the System power off does not drive the gpio expander
which might control the fan power supply.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>