Add support for _PDC to the ACPI processor "Performance States library" 
(perflib). If this field is empty, a bogus entry is passed to the _PDC
method so that the default (io) access is returned again. This patch
is partly based on David Moore's patch to
arch/i386/kernel/cpu/cpufreq/acpi.c, sent to the cpufreq mailing list on
June 24th, 2003.