ARM: shmobile: bockw dts: Override #sound-dai-cells to zero

The default value of #sound-dai-cells in r8a7778.dtsi is one, while
the /sound/simple-audio-card,cpu device node in r8a7778-bockw.dts uses a
phandle without any extra cells ("<&rcar_sound>"), causing:

    /sound/simple-audio-card,cpu: arguments longer than property
    asoc-simple-card sound: parse error -22
    asoc-simple-card: probe of sound failed with error -22

Override #sound-dai-cells to zero to fix this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>