dt-bindings: serial: fsl-lpuart: Fix i.MX 8QM compatible matching

The i.MX 8QM DTS files use two compatibles, so update the binding to fix
dtbs_check warnings like:

  arch/arm64/boot/dts/freescale/imx8qm-mek.dt.yaml: serial@5a060000:
    compatible: ['fsl,imx8qm-lpuart', 'fsl,imx8qxp-lpuart'] is too long

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1639680494-23183-2-git-send-email-abel.vesa@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/fsl-lpuart.yaml

@@ -27,6 +27,9 @@ properties:
               - fsl,imx8qxp-lpuart
               - fsl,imx8ulp-lpuart
           - const: fsl,imx7ulp-lpuart
+      - items:
+          - const: fsl,imx8qm-lpuart
+          - const: fsl,imx8qxp-lpuart
 
   reg:
     maxItems: 1