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>

FatPkg/FatPei/FatPei.inf

@@ -31,6 +31,7 @@
 
 [Sources]
   Mbr.c
+  Gpt.c
   Eltorito.c
   Part.c
   FatLiteApi.c
@@ -49,6 +50,7 @@
 [LibraryClasses]
   PcdLib
   BaseMemoryLib
+  MemoryAllocationLib
   PeimEntryPoint
   BaseLib
   DebugLib
@@ -61,6 +63,7 @@
   gRecoveryOnFatIdeDiskGuid                   ## SOMETIMES_CONSUMES   ## UNDEFINED
   gRecoveryOnFatFloppyDiskGuid                ## SOMETIMES_CONSUMES   ## UNDEFINED
   gRecoveryOnFatNvmeDiskGuid                  ## SOMETIMES_CONSUMES   ## UNDEFINED
+  gEfiPartTypeUnusedGuid                      ## SOMETIMES_CONSUMES   ## UNDEFINED
 
 
 [Ppis]