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>

arch/arm/boot/dts/imx6sx.dtsi

@@ -467,7 +467,7 @@ flexcan2: can@2094000 {
 			};
 
 			gpt: gpt@2098000 {
-				compatible = "fsl,imx6sx-gpt", "fsl,imx31-gpt";
+				compatible = "fsl,imx6sx-gpt", "fsl,imx6dl-gpt";
 				reg = <0x02098000 0x4000>;
 				interrupts = <GIC_SPI 55 IRQ_TYPE_LEVEL_HIGH>;
 				clocks = <&clks IMX6SX_CLK_GPT_BUS>,