ARM: dts: imx6dl-prtvt7: Remove unused 'sound-dai-cells' from ssi1 node

The 'fsl,ssi' documentation doesn't say anything about specifying this.

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>