ARM: dts: imx6sl/sll: Add the "fsl,imx6dl-gpt" fallback

ARM: dts: imx6sl/sll: Add the "fsl,imx6dl-gpt" fallback

Per fsl,imxgpt.yaml, the i.MX6SL GPT compatible should be described as:

compatible = "fsl,imx6sl-gpt", "fsl,imx6dl-gpt";

Do as suggested to fix the following dt-schema warnings:

timer@2098000: compatible: 'oneOf' conditional failed, one must be fixed:
	['fsl,imx6sl-gpt'] is too short
	'fsl,imx1-gpt' was expected
	'fsl,imx21-gpt' was expected
	'fsl,imx27-gpt' was expected
	'fsl,imx31-gpt' was expected
	'fsl,imx6sl-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,imx6sl-gpt' is not one of ['fsl,imx6ul-gpt', 'fsl,imx7d-gpt']

Signed-off-by: Fabio Estevam <[email protected]>
Signed-off-by: Shawn Guo <[email protected]>