ARM: dts: imx6sx: correct backward compatible of gpt

i.MX6SX has same GPT type as i.MX6DL, in GPT driver, it uses
below TIMER_OF_DECLARE, so the backward compatible should be
"fsl,imx6dl-gpt", correct it.

TIMER_OF_DECLARE(imx6sx_timer, "fsl,imx6sx-gpt", imx6dl_timer_init_dt);

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>