dt-bindings: trigger-source: add ADI Util Sigma-Delta SPI

dt-bindings: trigger-source: add ADI Util Sigma-Delta SPI

Add new binding for the ADI Util Sigma-Delta SPI FPGA IP Core.

This is used to trigger a SPI offload based on a RDY signal from the
ADC while masking out other signals on the same line.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20250627-iio-adc-ad7173-add-spi-offload-support-v2-8-f49c55599113@baylibre.com
Signed-off-by: Mark Brown <broonie@kernel.org>