hwmon: (it87) Add support for IT8790E

IT8790E is a super-IO chip with three fan tachometers. It is mostly
compatible to IT8728F, but only supports three fan tachometers
instead of five.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>