ARM: dts: imx6q: name iomuxc sub-nodes following pin function

Name iomuxc sub-nodes following pin function and hardware manual.
Signed-off-by: NShawn Guo <shawn.guo@linaro.org>

arch/arm/boot/dts/imx6q-sabrelite.dts

@@ -99,7 +99,7 @@
 			uart2: serial@021e8000 {
 				status = "okay";
 				pinctrl-names = "default";
-				pinctrl-0 = <&pinctrl_serial2_1>;
+				pinctrl-0 = <&pinctrl_uart2_1>;
 			};
 
 			i2c@021a0000 { /* I2C1 */

arch/arm/boot/dts/imx6q.dtsi

@@ -557,8 +557,8 @@
 					};
 				};
 
-				serial2 {
-					pinctrl_serial2_1: serial2grp-1 {
+				uart2 {
+					pinctrl_uart2_1: uart2grp-1 {
 						fsl,pins = <
 							183 0x1b0b1	/* MX6Q_PAD_EIM_D26__UART2_TXD */
 							191 0x1b0b1	/* MX6Q_PAD_EIM_D27__UART2_RXD */