dt-bindings:iio:dac: add ad7293 doc

Add device tree bindings for the AD7293 Power Amplifier.

Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211202150819.24832-2-antoniu.miclaus@analog.com


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