[CPUFREQ] p4-clockmod: fix support for Core

Support for Core CPUs was broken in two ways in speedstep-lib: for x86_64,
we missed a MSR definition; for both x86_64 and i386, the FSB calculation
was wrong by four (it's a quad-pumped bus). Also increase the accuracy
of the calculation.

Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>
Signed-off-by: Dave Jones <davej@redhat.com>