dt-bindings: timer: add imx7d compatible

Currently the dtbs_check for imx6ul generates warnings like this:

['fsl,imx7d-gpt', 'fsl,imx6sx-gpt'] is too long

The driver has no special handling for fsl,imx7d-gpt, so fsl,imx6sx-gpt is
used. Therefore make imx7d GPT compatible to the imx6sx one to fix the
warning.
Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20231012080033.2715241-4-alexander.stein@ew.tq-group.com

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

@@ -36,7 +36,9 @@ properties:
               - fsl,imxrt1170-gpt
           - const: fsl,imx6dl-gpt
       - items:
-          - const: fsl,imx6ul-gpt
+          - enum:
+              - fsl,imx6ul-gpt
+              - fsl,imx7d-gpt
           - const: fsl,imx6sx-gpt
 
   reg: