ASoC: dt-bindings: fsl_rpmsg: Add compatible string for i.MX8ULP

Add compatible string for i.MX8ULP platform which support audio
function through rpmsg audio channel on M core.
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1630553525-25655-1-git-send-email-shengjiu.wang@nxp.comSigned-off-by: Mark Brown <broonie@kernel.org>

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

@@ -21,6 +21,7 @@ properties:
       - fsl,imx8mn-rpmsg-audio
       - fsl,imx8mm-rpmsg-audio
       - fsl,imx8mp-rpmsg-audio
+      - fsl,imx8ulp-rpmsg-audio
 
   model:
     $ref: /schemas/types.yaml#/definitions/string