hwmon: (it87) Add support for IT8781F

IT8781F is mostly compatible to IT8782F. Major difference is that it only
supports four instead of six UART channels, and therefore does not share
the uart6 pins.

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