dt-bindings: timer: fsl,imxgpt: Fix the fsl,imx7d-gpt fallback

imx7s.dtsi correctly describes the GPT node as:

compatible = "fsl,imx7d-gpt", "fsl,imx6dl-gpt";

Document the fallback compatible to be "fsl,imx6dl-gpt" in the bindings.

This fixes the following dt-schema warnings:

timer@302f0000: compatible: 'oneOf' conditional failed, one must be fixed:
['fsl,imx7d-gpt', 'fsl,imx6dl-gpt'] is too long
'fsl,imx1-gpt' was expected
'fsl,imx21-gpt' was expected
'fsl,imx27-gpt' was expected
'fsl,imx31-gpt' was expected
'fsl,imx7d-gpt' is not one of ['fsl,imx25-gpt', 'fsl,imx50-gpt', 'fsl,imx51-gpt', 'fsl,imx53-gpt', 'fsl,imx6q-gpt']
'fsl,imx6dl-gpt' was expected
'fsl,imx7d-gpt' is not one of ['fsl,imx6sl-gpt', 'fsl,imx6sx-gpt', 'fsl,imx8mp-gpt', 'fsl,imxrt1050-gpt', 'fsl,imxrt1170-gpt']
'fsl,imx6sx-gpt' was expected

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20241202132147.587799-1-festevam@gmail.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>