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>