MdePkg/UefiSpec.h: Add definition to support Capsule-on-Disk feature

MdePkg/UefiSpec.h: Add definition to support Capsule-on-Disk feature

BZ:https://bugzilla.tianocore.org/show_bug.cgi?id=1482

UEFI Spec define this definition to support Capsule-on-Disk.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Zhang Chao B <chao.b.zhang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chen A Chen <chen.a.chen@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>