dt-bindings: iio: dac: add docs for ad8460

This adds the bindings documentation for the 14-bit
High Voltage, High Current, Waveform Generator
Digital-to-Analog converter.

Signed-off-by: Mariel Tinaco <Mariel.Tinaco@analog.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://patch.msgid.link/20240912095435.18639-2-Mariel.Tinaco@analog.com


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