cpufreq: tegra20: Use resource-managed API

Switch cpufreq-tegra20 driver to use resource-managed API.
This removes the need to get opp_table pointer using
dev_pm_opp_get_opp_table() in order to release OPP table that
was requested by dev_pm_opp_set_supported_hw(), making the code
a bit more straightforward.

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