hwmon: (it87) Add feature flag for AVCC3 support

AVCC3 is supported on IT8620E, similar to IT8603E. Add feature flag
to indicate AVCC3 support. Don't enable it for now on IT8620E since
it is unclear if this chip supports it correctly.

Tested-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>