hwmon: (pwm-fan) Add RPM support via external interrupt

This adds RPM support to the pwm-fan driver in order to use with
fancontrol/pwmconfig. This feature is intended for fans with a tachometer
output signal, which generate a defined number of pulses per revolution.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
[groeck: Drop unused 'devattr' variable]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>