UefiPayloadPkg: Align Identifier value with UPL spec

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3933

URL: https://universalpayload.github.io/documentation/
Currently, Identifier value is "UPLD", it needs to have correct value
"PLDH" based on Universal Payload Specification spec section 2.12.2

Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Guo Dong <guo.dong@intel.com>
Signed-off-by: Gua Guo <gua.guo@intel.com>

UefiPayloadPkg/UniversalPayloadBuild.py

@@ -29,7 +29,7 @@ class UPLD_INFO_HEADER(LittleEndianStructure):
         ]
 
     def __init__(self):
-        self.Identifier     =  b'UPLD'
+        self.Identifier     =  b'PLDH'
         self.HeaderLength   = sizeof(UPLD_INFO_HEADER)
         self.HeaderRevision = 0x0075
         self.Revision       = 0x0000010105