MdeModulePkg: Add PcdTdxSharedBitMask

RFC: https://bugzilla.tianocore.org/show_bug.cgi?id=3429

Guest Physical Address (GPA) space in Td guest is divided into private
and shared sub-spaces, determined by the SHARED bit of GPA. This PCD
holds the shared bit mask. Its default value is 0 and it will be set
in PlatformPei driver if it is of Td guest.

Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: Erdem Aktas <erdemaktas@google.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>

OvmfPkg/OvmfXen.dsc

@@ -495,6 +495,9 @@
   # Set memory encryption mask
   gEfiMdeModulePkgTokenSpaceGuid.PcdPteMemoryEncryptionAddressOrMask|0x0
 
+  # Set Tdx shared bit mask
+  gEfiMdeModulePkgTokenSpaceGuid.PcdTdxSharedBitMask|0x0
+
   gEfiSecurityPkgTokenSpaceGuid.PcdOptionRomImageVerificationPolicy|0x00
 
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