simple: add sound-dai-cells to I2S def

Add '#sound-dai-cells = <0>;' to the I2S definition in
bcm2708_common.dtsi

Not having it specified, whilst not causing an issue right now with
rpi-4.4.y, is going to cause an issue going forward with the use of
simple-card driver. So it doesn't fall through the cracks, patch it
in now.

Hopefully Martin has taken care of getting a patch submitted for the
upstream Pi dts, as it was he who first run into the issue with the
current upstream kernel....
msperl/linux-rpi#3 (comment)

Signed-off-by: DigitalDreamtime <clive.messer@digitaldreamtime.co.uk>

arch/arm/boot/dts/bcm2708_common.dtsi

@@ -137,6 +137,7 @@
 
  i2s: i2s@7e203000 {
  compatible = "brcm,bcm2835-i2s";
+ #sound-dai-cells = <0>;
  reg = <0x7e203000 0x24>,
  <0x7e101098 0x08>;