dt-bindings: iio: dac: ad5504: Add missing binding document

Binding for this high voltage DAC with temperature event signal.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20210627163244.1090296-8-jic23@kernel.org