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 <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

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

···
               - fsl,imx8qxp-lpuart
               - fsl,imx8ulp-lpuart
           - const: fsl,imx7ulp-lpuart
+      - items:
+          - const: fsl,imx8qm-lpuart
+          - const: fsl,imx8qxp-lpuart
 
   reg:
     maxItems: 1