ACPI: add "processor.ignore_ppc" hook to workaround BIOS _PPC weirdness

There have been fixes using _PPC, which seem to unhide a problem
on HP nx6125 (double cpufreq switch freezes the machine for
several seconds).

This one should provide a workaround for the nx6125 and for
possible other machines that show any weird _PPC behaviour.

Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>