[CPUFREQ] Sync p4-clockmod MSR access across logical CPUs.

[CPUFREQ] Sync p4-clockmod MSR access across logical CPUs.
As noted and debugged by Rutger Nijlunsing and verified in section 13.15.3 of Intel's
IA32 Intel Architecture Software Developer's Manual, Volume 3, the p4-clockmod
msr needs to be set to the same value on all logical CPUs ("siblings") to
function "properly".

This patch implements this, and uses cpufreq_p4_get instead of a local copy in
cpufreq_p4_setdc. The latter function now only does the actual setting, all
other (notification, verification and set_cpus_allowed()) stuff is done in
cpufreq_p4_target.