ARM: dts: imx6dl: Fix gpt compatibles, remove imx6q-gpt

imx6qdl.dtsi uses compatibles "fsl,imx6q-gpt", "fsl,imx31-gpt".
imx6dl.dtsi uses compatibles "fsl,imx6dl-gpt", "fsl,imx6q-gpt" since
commit

	4e415ed8 (ARM: dts: imx6dl: add imx6dl gpt specific compatible string)

If imx6dl would be compatible with imx6q-gpt it would also have to be
compatible with imx31-gpt which is currently missing.

Based on the above mentioned patch I assume imx6q-gpt and imx6dl-gpt are
not compatible. So imx6q-gpt should be removed as compatible.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>