MdeModulePkg: Add PcdTdxSharedBitMask

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>