dt-bindings: display: add #sound-dai-cells property to rockchip dw hdmi

The Rockchip DWC HDMI TX Encoder can take one I2S input and transmit it
over the HDMI output. Add #sound-dai-cells (= 0) to the binding for it.

Signed-off-by: Johan Jonker <jbx6244@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/3a035c16-75b5-471d-aa9d-e91c2bb9f8d0@gmail.com