cpufreq: tegra: Rename tegra-cpufreq to tegra20-cpufreq

The Tegra124 will use a different driver for frequency scaling, so
rename the old driver (which handles only Tegra20) appropriately.

Signed-off-by: Tuomas Tynkkynen <ttynkkynen@nvidia.com>
Signed-off-by: Mikko Perttunen <mikko.perttunen@kapsi.fi>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>