FatPkg: Add GPT check in FatPei to support Capsule-on-Disk feature.

FatPkg: Add GPT check in FatPei to support Capsule-on-Disk feature.

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=1470
This feature is used for finding GPT partition.
Follow the following step to check.
1) Check Protective MBR.
2) Check GPT primary/backup header.
3) Check GPT primary/backup entry array.

Cc: Ruiyu Ni <ray.ni@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: Hao Wu <hao.a.wu@intel.com>