Documentation/ABI/testing: Add documentation for AD5766 new ABI

New interface is proposed for dither functionality. This future allows
composing an external signals to the selected output channel.
The dither signal can be turned on/off, scaled, inverted, or it can be
selected from different sources.

Signed-off-by: Cristian Pop <cristian.pop@analog.com>
Link: https://lore.kernel.org/r/20210115112105.58652-2-cristian.pop@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>