dt-bindings: display: meson-dw-hdmi: add missing sound-name-prefix property

This is used in meson-gx and meson-g12. Add the property to the binding.
This fixes the dtschema warning:
hdmi-tx@c883a000: 'sound-name-prefix' does not match any of the
regexes: 'pinctrl-[0-9]+'

Signed-off-by: Alexander Stein <alexander.stein@mailbox.org>
Fixes: 376bf52dee ("dt-bindings: display: amlogic, meson-dw-hdmi: convert to yaml")
Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211223122434.39378-2-alexander.stein@mailbox.org

Documentation/devicetree/bindings/display/amlogic,meson-dw-hdmi.yaml

@@ -10,6 +10,9 @@ title: Amlogic specific extensions to the Synopsys Designware HDMI Controller
 maintainers:
   - Neil Armstrong <narmstrong@baylibre.com>
 
+allOf:
+  - $ref: /schemas/sound/name-prefix.yaml#
+
 description: |
   The Amlogic Meson Synopsys Designware Integration is composed of
   - A Synopsys DesignWare HDMI Controller IP
@@ -99,6 +102,8 @@ properties:
   "#sound-dai-cells":
     const: 0
 
+  sound-name-prefix: true
+
 required:
   - compatible
   - reg