dt-bindings: cpufreq: Add binding for NVIDIA Tegra20/30

Add device-tree binding that describes CPU frequency-scaling hardware
found on NVIDIA Tegra20/30 SoCs.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>