staging: iio: Documentation sysfs-bus-iio add power_mode

Add description about in_accelX_power_mode and
in_accel_power_mode_available.

Signed-off-by: Oleksandr Kravchenko <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/ABI/testing/sysfs-bus-iio

···
 		This attribute is used to read the amount of quadrature error
 		present in the device at a given time.
 
+What:		/sys/.../iio:deviceX/in_accelX_power_mode
+KernelVersion:	3.11
+Contact:	[e-mail]
+Description:
+		Specifies the chip power mode.
+		low_noise: reduce noise level from ADC,
+		low_power: enable low current consumption.
+		For a list of available output power modes read
+		in_accel_power_mode_available.
+
 What:		/sys/bus/iio/devices/iio:deviceX/store_eeprom
 KernelVersion:	3.4.0
 Contact:	[e-mail]