ARM: dts: imx6qdl: vicut1: The sgtl5000 uses i2s not ac97

According to Documentation/devicetree/bindings/sound/fsl,ssi.txt
'fsl,mode' should be specified for AC97 mode only.

The 'fsl,ssi' documentation doesn't say anything about specifying
'sound-dai-cells' so we'll remove that as well.

Signed-off-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>