ARM: dts: imx7: Declare timers compatible with fsl,imx6dl-gpt

The timer nodes declare compatibility with "fsl,imx6sx-gpt", which
itself is compatible with "fsl,imx6dl-gpt". Switch the fallback
compatible from "fsl,imx6sx-gpt" to "fsl,imx6dl-gpt".

Fixes: 94967345 ("ARM: dts: add imx7d soc dtsi file")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Roland Hieber <rhi@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>