iio: ABI: add new DAC powerdown mode

Add a new powerdown mode for DACs with 7.7kohm and 32kohm resistor
to GND.

Reviewed-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Kim Seer Paller <kimseer.paller@analog.com>
Link: https://patch.msgid.link/20250429-togreg-v7-1-0af9c543b545@analog.com


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