[CPUFREQ] e_powersaver: Additional checks

Some systems are using 1,2Ghz@844mV processors running at 600MHz@796mV.
Try to detect such systems and don't touch anything on it. If CPU doesn't have
P-States in BIOS it should run at maximum frequency.
Allow user to bypass checks by means of two new options.
Don't set frequency to maximum on module unloading to avoid bada boom.
It is also possible that some processors may have incorrect values in min/max
registers caused by error in manufacturing process. Probably it would be BIOS
job to set them to right frequency and P-States tables would have correct
values inside.
Two additional sanity checks for voltage.

Signed-off-by: Rafał Bilski <rafalbilski@interia.pl>
Signed-off-by: Dave Jones <davej@redhat.com>