SecurityPkg: Add gEdkiiSecureBootModeGuid definition

SecurityPkg: Add gEdkiiSecureBootModeGuid definition

Add gEdkiiSecureBootModeGuid definition for Enable Secure Boot feature defined in
UEFI2.5 Mantis 1263. It is a private variable GUID.
  https://mantis.uefi.org/mantis/view.php?id=1263

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Chao Zhang <chao.b.zhang@intel.com>
Reviewed-by: Zeng Star <star.zeng@intel.com>
Reviewed-by: Long Qin <qin.long@intel.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19132 6f19259b-4bc3-4df7-8a09-765794883524