IT8620E is mostly compatible to IT7828F. Add generic support for it.

IT8620E supports up to 6 fan tachometers and 6 pwm controls.
Support for the 6th tachometer and for the additional pwm controls
are addded in separate patches.
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>