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

imx6qdl-icore-rqs 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.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>