iio: ABI: add DAC 42kohm_to_gnd powerdown mode

Add a new powerdown mode for DACs with 42kohm resistor to GND.

Signed-off-by: Kim Seer Paller <kimseer.paller@analog.com>
Link: https://patch.msgid.link/20240718051834.32270-3-kimseer.paller@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>