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>

Documentation/hwmon/sysfs-interface

@@ -505,6 +505,15 @@ energy[1-*]_input		Cumulative energy use
 				RO
 
 
+************
+* Humidity *
+************
+
+humidity[1-*]_input		Humidity
+				Unit: milli-percent (per cent mille, pcm)
+				RO
+
+
 **********
 * Alarms *
 **********