hwmon: (it87) Add IT8728F support

Until we get a datasheet for the IT8728F, treat it as fully compatible
with the IT8721F, as it seems to work reasonably well.

This closes kernel bug #27262.

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