dt-bindings: iio: addac: adi,ad74413r: use spi-peripheral-props.yaml

Reference the spi-peripheral-props.yaml schema to allow using all
properties typical for SPI-connected devices, even these which device
bindings author did not tried yet.

While changing additionalProperties->unevaluatedProperties, put it in
typical place, just before example DTS.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20221004115642.63749-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>