UefiCpuPkg: Add new PCDs PROMPT/HELP string in UNI file

Correct PCD declaration comments and add new PCDs in UNI file.

Cc: Feng Tian <feng.tian@intel.com>
Cc: Michael 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>