dt-bindings: mailbox: fsl,mu: Fix i.MX 8QXP compatible matching

The Mailbox on i.MX 8QXP (fsl,imx8qxp-mu) can also be compatible with
fsl,imx8-mu-scu (for fast IPC) so adjust the compatibles to fix
dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8qxp-mek.dt.yaml: mailbox@5d1f0000:
    compatible: ['fsl,imx8-mu-scu', 'fsl,imx8qxp-mu', 'fsl,imx6sx-mu']
    is not valid under any of the given schemas (Possible causes of the failure):

  arch/arm64/boot/dts/freescale/imx8qxp-mek.dt.yaml: mailbox@5d1f0000:
    compatible: ['fsl,imx8-mu-scu', 'fsl,imx8qxp-mu', 'fsl,imx6sx-mu'] is too long
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200903195325.5394-1-krzk@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

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

@@ -38,8 +38,9 @@ properties:
           - const: fsl,imx6sx-mu
       - description: To communicate with i.MX8 SCU with fast IPC
         items:
-          - const: fsl,imx8qxp-mu
           - const: fsl,imx8-mu-scu
+          - const: fsl,imx8qxp-mu
+          - const: fsl,imx6sx-mu
 
   reg:
     maxItems: 1