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

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.org
Signed-off-by: Rob Herring <robh@kernel.org>