UefiCpuPkg/CpuDxe: Fix hard code actual TimerPeriod value

UefiCpuPkg/CpuDxe: Fix hard code actual TimerPeriod value

Current CpuGetTimerValue() implementation return hard code TimerPeriod value. We
could calculate the actual TimerPeriod value over period of time (100us) at the
first time invoking CpuGetTimerValue() and save the TimerPeriod value into one
global variable to avoid delay at the next CpuGetTimerValue() invoking.

https://bugzilla.tianocore.org/show_bug.cgi?id=382

Cc: Feng Tian <feng.tian@intel.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jeff Fan <jeff.fan@intel.com>
Reviewed-by: Feng Tian <feng.tian@Intel.com>