dt-bindings: mailbox: imx-mu: add i.MX8ULP MU support

The register layout and bits definition of i.MX8ULP MU is different
compared with others, let's add the compatible for the new MU.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

Documentation/devicetree/bindings/mailbox/fsl,mu.yaml

@@ -26,6 +26,7 @@ properties:
     oneOf:
       - const: fsl,imx6sx-mu
       - const: fsl,imx7ulp-mu
+      - const: fsl,imx8ulp-mu
       - const: fsl,imx8-mu-scu
       - items:
           - enum: