hwmon: (it87) Add support for old automatic fan speed control

Add support for the automatic fan speed control interface as
implemented by IT8705F chips up to revision F and IT8712F chips up to
revision G. This implementation fits very well in our standard sysfs
interface.

I implemented the old and not the new interface because the only chip
I have at hand is an old one, and the new interface is more difficult
to map to the standard sysfs interface. Adding support later should be
possible though, if someone with a supported chip is interested.

Signed-off-by: Jean Delvare <khali@linux-fr.org>