UefiCpuPkg/MicrocodeUpdate: enhance flash write logic

UefiCpuPkg/MicrocodeUpdate: enhance flash write logic

The patch updated MicrocodeWrite() to move the Microcode replacement logic
to a standalone function -  UpdateMicrocodeFlashRegion().
More detail description is added in UpdateMicrocodeFlashRegion()
to improve readability.

The Microcode information is collected in InitializeMicrocodeDescriptor(),
so that FmpGetImage() can get the info directly.
MicrocodeRead() is not needed any more.

Cc: Jeff Fan <jeff.fan@intel.com>
Cc: Star Zeng <star.zeng@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jiewen Yao <jiewen.yao@intel.com>
Reviewed-by: Jeff Fan <jeff.fan@intel.com>