dt-bindings: iio: dac: Add adi,ad3552r.yaml

Add documentation for ad3552r and ad3542r

Signed-off-by: Mihail Chindris <mihail.chindris@analog.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>