ARM: dts: i.MX6qdl: Add i.MX31 compatible to gpt node

The i.MX6 gpt is handled by the i.MX31 gpt driver in the kernel,
so add a corresponding compatible entry.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>

arch/arm/boot/dts/imx6qdl.dtsi

@@ -294,7 +294,7 @@ can2: flexcan@02094000 {
 			};
 
 			gpt: gpt@02098000 {
-				compatible = "fsl,imx6q-gpt";
+				compatible = "fsl,imx6q-gpt", "fsl,imx31-gpt";
 				reg = <0x02098000 0x4000>;
 				interrupts = <0 55 0x04>;
 				clocks = <&clks 119>, <&clks 120>;