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 <[e-mail]>
Signed-off-by: Oleksij Rempel <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

arch/arm/boot/dts/imx6dl-prtvt7.dts

···
 };
 
 &ssi1 {
-	#sound-dai-cells = <0>;
 	status = "okay";
 };