iio: ABI: Add DAC 500ohm, 3.85kohm, and 16kohm powerdown modes

Add powerdown mode entries for DACs with 500 Ohm, 3.85 kOhm, and
16 kOhm resistor to ground output impedance states. These are used
by the AD3531/AD3531R 4-channel DAC.

Signed-off-by: Kim Seer Paller <kimseer.paller@analog.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Author: kseerp

Documentation/ABI/testing/sysfs-bus-iio

@@ -764,10 +764,13 @@ Contact:	linux-iio@vger.kernel.org
 Description:
 		Specifies the output powerdown mode.
 		DAC output stage is disconnected from the amplifier and
+		500ohm_to_gnd: connected to ground via a 500Ohm resistor,
 		1kohm_to_gnd: connected to ground via an 1kOhm resistor,
 		2.5kohm_to_gnd: connected to ground via a 2.5kOhm resistor,
+		3.85kohm_to_gnd: connected to ground via a 3.85kOhm resistor,
 		6kohm_to_gnd: connected to ground via a 6kOhm resistor,
 		7.7kohm_to_gnd: connected to ground via a 7.7kOhm resistor,
+		16kohm_to_gnd: connected to ground via a 16kOhm resistor,
 		20kohm_to_gnd: connected to ground via a 20kOhm resistor,
 		32kohm_to_gnd: connected to ground via a 32kOhm resistor,
 		42kohm_to_gnd: connected to ground via a 42kOhm resistor,