dt-bindings: dsp: fsl: Add SOF compatile string for i.MX8ULP

Add SOF compatile string "fsl,imx8ulp-dsp" for supporting DSP
device on i.MX8ULP platform.

Signed-off-by: Shengjiu Wang <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Reviewed-by: Daniel Baluta <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mark Brown <[e-mail]>

Documentation/devicetree/bindings/dsp/fsl,dsp.yaml

···
       - fsl,imx8qxp-dsp
       - fsl,imx8qm-dsp
       - fsl,imx8mp-dsp
+      - fsl,imx8ulp-dsp
       - fsl,imx8qxp-hifi4
       - fsl,imx8qm-hifi4
       - fsl,imx8mp-hifi4