ARM: dts: imx6sl-evk: add missing GPIO iomux setting

On i.MX6SL EVK board, the MX6SL_PAD_KEY_ROW5 pin is
used as lcd 3v3 regulator control pin, need to make
sure MX6SL_PAD_KEY_ROW5 is muxed as GPIO function
for controlling lcd 3v3 regulator.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by; Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6sl-evk.dts

@@ -77,6 +77,8 @@ reg_aud4v: regulator-aud4v {
 
 	reg_lcd_3v3: regulator-lcd-3v3 {
 		compatible = "regulator-fixed";
+		pinctrl-names = "default";
+		pinctrl-0 = <&pinctrl_reg_lcd_3v3>;
 		regulator-name = "lcd-3v3";
 		gpio = <&gpio4 3 GPIO_ACTIVE_HIGH>;
 		enable-active-high;
@@ -410,6 +412,12 @@ MX6SL_PAD_PWM1__PWM1_OUT 0x110b0
 			>;
 		};
 
+		pinctrl_reg_lcd_3v3: reglcd3v3grp {
+			fsl,pins = <
+				MX6SL_PAD_KEY_ROW5__GPIO4_IO03    0x17059
+			>;
+		};
+
 		pinctrl_uart1: uart1grp {
 			fsl,pins = <
 				MX6SL_PAD_UART1_RXD__UART1_RX_DATA	0x1b0b1