cpufreq: ACPI / CPPC: Add module support for cppc_cpufreq driver

Add a function to cleanup at module exit and export
appropriate GPL string to enable moduler support
for the cppc_cpufreq driver.

Reported-by: Srinivas Pandruvada <srinivas.pandruvada@intel.com>
Signed-off-by: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>