MdePkg: Add Tdx.h

MdePkg: Add Tdx.h

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

Tdx.h includes the Intel Trust Domain Extension definitions.

Detailed information can be found in below document:
https://software.intel.com/content/dam/develop/external/us/en/
documents/tdx-module-1eas-v0.85.039.pdf

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Zhiguang Liu <zhiguang.liu@intel.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>