[CPUFREQ] Recalibrate cpu_khz [1/2]

We have to recalibrate cpu_khz in order to use the current FID instead the max
FID since some BIOS do not put the processor at maximum frequency at POST. 
Also, some BIOS will change the processor frequency at our back after cpu_khz
was calibrate.  Finally, this will fix a long standing bug when we do
something like this:

# rmmod powernow-k7
# modprobe powernow-k7

Signed-off-by: Bruno Ducrot <ducrot@poupinou.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Dave Jones <davej@redhat.com>