ASoC: dt-bindings: fsl,spdif: Add compatible string for imx8ulp

Add compatible string for imx8ulp, which supports spdif module
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1622613849-10271-1-git-send-email-shengjiu.wang@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/fsl,spdif.yaml

@@ -25,6 +25,7 @@ properties:
       - fsl,imx8mq-spdif
       - fsl,imx8mm-spdif
       - fsl,imx8mn-spdif
+      - fsl,imx8ulp-spdif
 
   reg:
     maxItems: 1