Allow for per-CPU initialization of CPUfreq. Therefore, it's not
necessary any longer to kmalloc the per-CPU policy struct. To use
this, cpufreq_driver->policy has to be set to NULL. Of course,
cpufreq_driver->init is needed then, which is the appropriate function
for CPU initialization. cpufreq_driver->exit is available for cleanup.

All existing drivers continue to work without any changes, just for
clarity ->init and ->exit are set to NULL, and the names accordingly.