cpufreq: tegra20: Allow cpufreq driver to be built as loadable module

Nothing prevents Tegra20 CPUFreq module to be unloaded, hence allow it to
be built as a non-builtin kernel module.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>