hwmon: (it87) Add feature flags for fans count and 16-bit fan configuration

Fans 4-5 are not supported on all chips and revisions. Also, 16-bit fan
counters are always enabled on some chips. Provide feature flags to
simplify adding support for new chips.

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