staging: iio: ad5933: add ABI documentation

Add an ABI documentation for the ad5933 driver.

Signed-off-by: Marcelo Schmitt <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/staging/iio/Documentation/sysfs-bus-iio-impedance-analyzer-ad5933 Documentation/ABI/testing/sysfs-bus-iio-impedance-analyzer-ad5933

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-What:		/sys/bus/iio/devices/iio:deviceX/outY_freq_start
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_frequency_start
+Date:		March 2019
 KernelVersion:	3.1.0
 Contact:	[e-mail]
 Description:
 		Frequency sweep start frequency in Hz.
 
-What:		/sys/bus/iio/devices/iio:deviceX/outY_freq_increment
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_frequency_increment
+Date:		March 2019
 KernelVersion:	3.1.0
 Contact:	[e-mail]
 Description:
 		Frequency increment in Hz (step size) between consecutive
 		frequency points along the sweep.
 
-What:		/sys/bus/iio/devices/iio:deviceX/outY_freq_points
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_frequency_points
+Date:		March 2019
 KernelVersion:	3.1.0
 Contact:	[e-mail]
 Description:
 		Number of frequency points (steps) in the frequency sweep.
-		This value, in conjunction with the outY_freq_start and the
-		outY_freq_increment, determines the frequency sweep range
-		for the sweep operation.
+		This value, in conjunction with the
+		out_altvoltageY_frequency_start and the
+		out_altvoltageY_frequency_increment, determines the frequency
+		sweep range for the sweep operation.
 
-What:		/sys/bus/iio/devices/iio:deviceX/outY_settling_cycles
+What:		/sys/bus/iio/devices/iio:deviceX/out_altvoltageY_settling_cycles
+Date:		March 2019
 KernelVersion:	3.1.0
 Contact:	[e-mail]
 Description: