ARM: dts: imx6dl: add imx6dl gpt specific compatible string

The i.MX6DL/S GPT has a different programming model from i.MX6Q one.
Add the compatible string "fsl,imx6dl-gpt" for it, and leave
"fsl,imx6q-gpt" there to keep the existing/old kernel happy.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>