iio: hid: Add humidity sensor support

Environmental humidity sensor is a hid defined sensor,
it shows raw humidity measurement of air.

More information can be found in:
http://www.usb.org/developers/hidpage/HUTRR39b.pdf

According to IIO ABI definition, humidityrelative data output unit is
milli percent. Add the unit convert from percent to milli percent.

Signed-off-by: Song Hongyan <hongyan.song@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/include/linux/hid-sensor-ids.h b/include/linux/hid-sensor-ids.h
index 46dd1f2..761f862 100644
--- a/include/linux/hid-sensor-ids.h
+++ b/include/linux/hid-sensor-ids.h
@@ -49,6 +49,10 @@
 #define	HID_USAGE_SENSOR_TEMPERATURE				0x200033
 #define	HID_USAGE_SENSOR_DATA_ENVIRONMENTAL_TEMPERATURE		0x200434
 
+/* humidity */
+#define HID_USAGE_SENSOR_HUMIDITY                              0x200032
+#define HID_USAGE_SENSOR_ATMOSPHERIC_HUMIDITY                  0x200433
+
 /* Gyro 3D: (200076) */
 #define HID_USAGE_SENSOR_GYRO_3D				0x200076
 #define HID_USAGE_SENSOR_DATA_ANGL_VELOCITY			0x200456