iio: Add ABI documentation for RGBC intensity hardware gain factor

This patch adds documentation for RGBC in_intensity_*_hardwaregain.
There is at least one user for these, ADJD-S311-CR999 digital color sensor
driver.

Signed-off-by: Roberta Dobrescu <roberta.dobrescu@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -337,6 +337,10 @@ Description:
 		are listed in this attribute.
 
 What		/sys/bus/iio/devices/iio:deviceX/out_voltageY_hardwaregain
+What:		/sys/bus/iio/devices/iio:deviceX/in_intensity_red_hardwaregain
+What:		/sys/bus/iio/devices/iio:deviceX/in_intensity_green_hardwaregain
+What:		/sys/bus/iio/devices/iio:deviceX/in_intensity_blue_hardwaregain
+What:		/sys/bus/iio/devices/iio:deviceX/in_intensity_clear_hardwaregain
 KernelVersion:	2.6.35
 Contact:	linux-iio@vger.kernel.org
 Description: