ASoC: dt-bindings: Add binding for Tegra20 S/PDIF

Add device-tree binding for Tegra20 S/PDIF controller.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20211204143725.31646-2-digetx@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>