cpufreq: tegra20: Remove EMC clock usage

The EMC driver has been gone 4 years ago, since the commit a7cbe92c
("ARM: tegra: remove tegra EMC scaling driver"). Remove the EMC clock
usage as it does nothing. We may consider re-implementing the EMC scaling
later, probably using PM Memory Bandwidth QoS API.

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>