dt-bindings: serial: fsl-lpuart: Fix compatible matching

The i.MX 8QXP DTSes use two compatibles so update the binding to fix
dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8qxp-mek.dt.yaml: serial@5a060000:
    compatible: ['fsl,imx8qxp-lpuart', 'fsl,imx7ulp-lpuart'] is too long
    From schema: Documentation/devicetree/bindings/serial/fsl-lpuart.yaml

  arch/arm64/boot/dts/freescale/imx8qxp-mek.dt.yaml: serial@5a060000:
    compatible: Additional items are not allowed ('fsl,imx7ulp-lpuart' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>