hwmon: Add humidity attribute to sysfs ABI

The humidity attribute is already supported by the sht15 driver, and another
driver supporting it is about to be added. Make it official.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Jean Delvare <khali@linux-fr.org>