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>

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

@@ -14,13 +14,16 @@ allOf:
 
 properties:
   compatible:
-    enum:
+    oneOf:
+      - enum:
           - fsl,vf610-lpuart
           - fsl,ls1021a-lpuart
           - fsl,ls1028a-lpuart
           - fsl,imx7ulp-lpuart
-      - fsl,imx8qxp-lpuart
           - fsl,imx8qm-lpuart
+      - items:
+          - const: fsl,imx8qxp-lpuart
+          - const: fsl,imx7ulp-lpuart
 
   reg:
     maxItems: 1