[ACPI] acpi-cpufreq fixups from Dominik Brodowski

[ACPI] acpi-cpufreq fixups from Dominik Brodowski

- remove unnecessary usage of flags.performance
- remove double check of _PPC in acpi-cpufreq driver as it's handled in
  drivers/acpi/processor.c already
- remove unneeded EXPORT_SYMBOL
- allocation of memory only for probed CPUs
- add unregistration function to the core
- fix OOPS when PST has core_frequency values of zero
- fix cpufreq_get() output
- fix /proc/acpi/processor/*/performance write support [deprecated]