ARM: dts: imx6ul-14x14-evk: Fix SPI chipselect polarity

The GPIO expander SPI chipselect is active low. Mark it as such to avoid
the following warning:

[    6.839213] gpio@0 enforce active low on chipselect handle
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/imx6ul-14x14-evk.dtsi

@@ -101,7 +101,7 @@ spi4 {
 		status = "okay";
 		gpio-sck = <&gpio5 11 0>;
 		gpio-mosi = <&gpio5 10 0>;
-		cs-gpios = <&gpio5 7 0>;
+		cs-gpios = <&gpio5 7 GPIO_ACTIVE_LOW>;
 		num-chipselects = <1>;
 		#address-cells = <1>;
 		#size-cells = <0>;