ASoC: dt-bindings: spdif-dit: add missing sound-name-prefix property

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

Signed-off-by: Alexander Stein <alexander.stein@mailbox.org>
Link: https://lore.kernel.org/r/20211223122434.39378-3-alexander.stein@mailbox.org
Signed-off-by: Mark Brown <broonie@kernel.org>