cpufreq: tegra20: Use generic cpufreq-dt driver (Tegra30 supported now)

Re-parenting to intermediate clock is supported now by the clock driver
and thus there is no need in a customized CPUFreq driver, all that code
is common for both Tegra20 and Tegra30. The available CPU freqs are now
specified in device-tree in a form of OPPs, all users should update their
device-trees.

Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Tested-by: Peter Geis <pgwipeout@gmail.com>
Tested-by: Marcel Ziswiler <marcel@ziswiler.com>
Tested-by: Jasper Korten <jja2000@gmail.com>
Tested-by: David Heidelberg <david@ixit.cz>
Tested-by: Nicolas Chauvet <kwizart@gmail.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>