ARM: dts: imx6sx-nitrogen6sx: drop incorrect cpu-dai property

drop incorrect cpu-dai property, change it to ssi-controller
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

arch/arm/boot/dts/nxp/imx/imx6sx-nitrogen6sx.dts

@@ -83,7 +83,7 @@ reg_wlan: regulator-wlan {
 	sound {
 		compatible = "fsl,imx-audio-sgtl5000";
 		model = "imx6sx-nitrogen6sx-sgtl5000";
-		cpu-dai = <&ssi1>;
+		ssi-controller = <&ssi1>;
 		audio-codec = <&codec>;
 		audio-routing =
 			"MIC_IN", "Mic Jack",