ASoC: meson: g12a: add missing sound-name-prefix property

This is used in meson-sm1 and meson-g12 .dtsi. Add the property to
the binding.
This fixes the dtschema warning:
audio-controller@740: 'sound-name-prefix' does not match any of the
regexes: 'pinctrl-[0-9]+'

Signed-off-by: Alexander Stein <alexander.stein@mailbox.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211218143423.18768-3-alexander.stein@mailbox.org
Signed-off-by: Mark Brown <broonie@kernel.org>