ARM: dts: imx6ul: segin: Fix stmpe touchscreen subnode name

The touchscreen subnode name needs to be stmpe_touchscreen as mentioned
in the dt-bindings.
Signed-off-by: Stefan Riedmueller <s.riedmueller@phytec.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6ul-phytec-segin.dtsi

@@ -139,7 +139,7 @@ stmpe: touchscreen@44 {
 		pinctrl-0 = <&pinctrl_stmpe>;
 		status = "disabled";
 
-		touchscreen {
+		stmpe_touchscreen {
 			compatible = "st,stmpe-ts";
 			st,sample-time = <4>;
 			st,mod-12b = <1>;