dt-bindings: display: bridge: it6505: Add #sound-dai-cells

The ITE IT6505 display bridge can take one I2S input and transmit it
over the DisplayPort link.

Add #sound-dai-cells (= 0) to the binding for it.

Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240327085250.3427496-1-wenst@chromium.org