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

Documentation/devicetree/bindings/display/bridge/adi,adv7533.yaml

@@ -9,6 +9,9 @@ title: Analog Devices ADV7533/35 HDMI Encoders
 maintainers:
   - Laurent Pinchart <laurent.pinchart@ideasonboard.com>
 
+allOf:
+  - $ref: /schemas/sound/dai-common.yaml#
+
 description: |
   The ADV7533 and ADV7535 are HDMI audio and video transmitters
   compatible with HDMI 1.4 and DVI 1.0. They support color space
@@ -89,6 +92,9 @@ properties:
     $ref: /schemas/types.yaml#/definitions/uint32
     enum: [ 1, 2, 3, 4 ]
 
+  "#sound-dai-cells":
+    const: 0
+
   ports:
     description:
       The ADV7533/35 has two video ports and one audio port.