hwmon: (adt7475) fan stall prevention

By default adt7475 will stop the fans (pwm duty cycle 0%) when the
temperature drops past Tmin - hysteresis. Some systems want to keep the
fans moving even when the temperature drops so add new sysfs attributes
that configure the enhanced acoustics min 1-3 which allows the fans to
run at the minimum configure pwm duty cycle.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>