dt-bindings: imxgpt: add imx6ul compatible

Currently the dtbs_check for imx6ul generates warnings like this:

['fsl,imx6ul-gpt', 'fsl,imx6sx-gpt'] is too long

According to the timer-imx-gpt driver all imx6 use the same imx6dl data,
but according to the existing DTS files the imx6ul GPT IP is derived from
imx6sx. So better follow the DTS files here and make the imx6ul GPT
compatible to the imx6sl one to fix the warning.
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230530100843.15072-3-stefan.wahren@i2se.comSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/timer/fsl,imxgpt.yaml

@@ -34,6 +34,9 @@ properties:
               - fsl,imxrt1050-gpt
               - fsl,imxrt1170-gpt
           - const: fsl,imx6dl-gpt
+      - items:
+          - const: fsl,imx6ul-gpt
+          - const: fsl,imx6sx-gpt
 
   reg:
     maxItems: 1