drm/tegra: hdmi: Register audio CODEC on Tegra20

Tegra20 SoC supports only S/PDIF source for HDMI audio. Register ASoC HDMI
S/PDIF CODEC for Tegra20, it will be linked with the S/PDIF CPU DAI.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>