ARM: tegra: Create platform device for tegra20-cpufreq driver

Tegra20-cpufreq driver require a platform device in order to be loaded,
instantiate a simple platform device for the driver during of the machines
late initialization.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>