base_mhz is calculated directly from the base_ratio
reported in MSR_NHM_PLATFORM_INFO * bclk,
and bclk is discovered via MSR or cpuid.
This reduces the dependency of Bzy_MHz calculation on the TSC.
Previously, there were 4 TSC readings required in each caculation,
the raw TSC delta combined with the measurement_interval.
This also removes the "tsc_tweak" correction factor used when
TSC runs on a different base clock from the CPU's bclk.
After this change, tsc_tweak is used only for %Busy.
The end-result should be a Bzy_MHz result slightly less prone to jitter.