ARM: dts: imx6qdl-rex: Remove unneeded 'fsl,mode' property

imx6qdl-rex boards use sgtl5000 codec and the machine file (imx-sgtl5000)
already sets SSI in slave mode and codec in master mode, so there is no need
for having this property.

Cc: Robert Nelson <robertcnelson@gmail.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>

arch/arm/boot/dts/imx6qdl-rex.dtsi

@@ -308,7 +308,6 @@
 };
 
 &ssi1 {
-	fsl,mode = "i2s-slave";
 	status = "okay";
 };