Currently, the TSC cpufreq notifiers does almost nothing on P4s, as we
assumed the TSC to be constant independent of _all_ frequency transitions.
Extensive testing by Karol Kozimor has shown, though, that only _throttling_
does not affect the TSC, but _scaling_ does.
                                                                                
So:
- pass the CPUFREQ_CONST_LOOPS flags (to be exact, all flags) to cpufreq
  transition notifiers
- skip TSC value changes if this flag is set
- set this flag for P4 / P4-Ms only in p4-clockmod [On Pentium-M banias
  the TSC _is_ affected by p4-clock modulation