ARM: tegra: asus-tf101: Enable S/PDIF and HDMI audio

Enable S/PDIF controller to enable HDMI audio support on ASUS TF101.
Use nvidia,fixed-parent-rate property that prevents audio rate conflict
between S/PDIF and I2S.

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