cpufreq: tegra124: Allow building as a module

This requires four changes:
* Using the cpufreq-dt register helper to establish a hard dependency
  for depmod to track
* Adding a remove routine to remove the cpufreq-dt device
* Adding a exit routine to handle cleaning up the driver
* Populating module license

Signed-off-by: Aaron Kling <webgeek1234@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>