dt-bindings: serial: fsl-imx-uart: fix i.MX 53 and 6 compatible matching

The i.MX 53 and i.MX6Q DTS use two compatibles, i.MX 6SL/6SLL/SX three
so update the binding to fix dtbs_check warnings like:

  serial@21ec000: compatible: ['fsl,imx6q-uart', 'fsl,imx21-uart'] is not valid under any of the given schemas
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200925212649.23183-1-krzk@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/serial/fsl-imx-uart.yaml

@@ -18,8 +18,6 @@ properties:
     oneOf:
       - const: fsl,imx1-uart
       - const: fsl,imx21-uart
-      - const: fsl,imx53-uart
-      - const: fsl,imx6q-uart
       - items:
           - enum:
             - fsl,imx25-uart
@@ -28,12 +26,18 @@ properties:
             - fsl,imx35-uart
             - fsl,imx50-uart
             - fsl,imx51-uart
+            - fsl,imx53-uart
+            - fsl,imx6q-uart
           - const: fsl,imx21-uart
       - items:
           - enum:
             - fsl,imx6sl-uart
             - fsl,imx6sll-uart
             - fsl,imx6sx-uart
+          - const: fsl,imx6q-uart
+          - const: fsl,imx21-uart
+      - items:
+          - enum:
             - fsl,imx6ul-uart
             - fsl,imx7d-uart
             - fsl,imx8mm-uart