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 <[e-mail]>
Signed-off-by: Krzysztof Kozlowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Rob Herring <[e-mail]>

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

···
     oneOf:
       - const: fsl,imx1-uart
       - const: fsl,imx21-uart
-      - const: fsl,imx53-uart
-      - const: fsl,imx6q-uart
       - items:
           - enum:
             - fsl,imx25-uart
···
             - 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