cpufreq: tegra20: Check if this is Tegra20 machine

Don't even try to request the clocks during of module initialization on
non-Tegra20 machines (this is the case for a multi-platform kernel) for
consistency.

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>