ARM: dts: at91: tse850: make the sound dai cell count explicit

The node is referred to as a DAI from the sound node and should
therefore have a #sound-dai-cells property.

Fixes: 21dd0ece

 ("ARM: dts: at91: add devicetree for the Axentia TSE-850")
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>