dt-bindings: iio: frequency: use spi-peripheral-props.yaml

For devices connectable by SPI bus (e.g. already using
"spi-max-frequency" property), reference the "spi-peripheral-props.yaml"
schema to allow using all SPI device properties, 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/20221004115642.63749-3-krzysztof.kozlowski@linaro.org
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>