hwmon: Add tachometer pulses per fan revolution to sysfs ABI

Some fan control chips support a configuration register to set the number of
tachometer pulses per fan revolution. Add an ABI attribute to support this
configuration register.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Jean Delvare <khali@linux-fr.org>