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: NShawn Guo <shawn.guo@linaro.org>

arch/arm/boot/dts/imx6dl.dtsi

@@ -106,6 +106,10 @@
 	};
 };
 
+&gpt {
+	compatible = "fsl,imx6dl-gpt", "fsl,imx6q-gpt";
+};
+
 &hdmi {
 	compatible = "fsl,imx6dl-hdmi";
 };