iio: Documentation: Add missing documentation for power attribute

Commit c43a102e ("iio: ina2xx: add support for TI INA2xx Power Monitors")
introduced the in_powerY_raw attribute, but omitted the corresponding
documentation.

The description is correct for the INA2xx and the MAX9611 IIO drivers.
Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Documentation/ABI/testing/sysfs-bus-iio

@@ -119,6 +119,15 @@ Description:
 		unique to allow association with event codes. Units after
 		application of scale and offset are milliamps.
 
+What:		/sys/bus/iio/devices/iio:deviceX/in_powerY_raw
+KernelVersion:	4.5
+Contact:	linux-iio@vger.kernel.org
+Description:
+		Raw (unscaled no bias removal etc.) power measurement from
+		channel Y. The number must always be specified and
+		unique to allow association with event codes. Units after
+		application of scale and offset are milliwatts.
+
 What:		/sys/bus/iio/devices/iio:deviceX/in_capacitanceY_raw
 KernelVersion:	3.2
 Contact:	linux-iio@vger.kernel.org