dt-bindings: display: adi,adv75xx: Document #sound-dai-cells

When using audio from ADV7533 or ADV7535 and describing the audio
card via simple-audio-card, the '#sound-dai-cells' needs to be passed.

Document the '#sound-dai-cells' property to fix the following
dt-schema warning:

imx8mn-beacon-kit.dtb: hdmi@3d: '#sound-dai-cells' does not match any of the regexes: 'pinctrl-[0-9]+'
	from schema $id: http://devicetree.org/schemas/display/bridge/adi,adv7533.yaml#



Signed-off-by: Fabio Estevam <festevam@denx.de>
Acked-by: Adam Ford <aford173@gmail.com>
Link: https://lore.kernel.org/r/20231206093643.2198562-1-festevam@gmail.com


Signed-off-by: Rob Herring <robh@kernel.org>