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: 376bf52d

 ("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