imx6qdl-gw5x 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: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>