ARM: dts: imx: remove ssi fsl,mode for audio cards

The DAI mode is and should be configured by the sound card driver as
codec and ssi have to be in the right modes to communicate with each
other. It is possible to operate the ssi unit or the codec in master mode,
sometimes even on the same board in different configurations.

With the latest changes in the fsl-ssi driver, the 'fsl,mode' property
is only handled as a fallback property. If the sound card sets the DAI
mode correctly, this fallback configuration is dropped.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@freescale.com>