MdePkg: Update Cpuid.h for Tdx

MdePkg: Update Cpuid.h for Tdx

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

Guest software can be designed to run either as a TD, as a legacy virtual
machine, or directly on the CPU, based on enumeration of its run-time
environment. [TDX-Module] Chap 10.2 defines the flow and the new CPUID
leaf 0x21.

[TDX-Module] Architecture Specification: Intel(R) Trust Domain Extensions
Module, Chap 10.2, 344425-003US, August 2021
https://www.intel.com/content/dam/develop/external/us/en/documents/
tdx-module-1.0-public-spec-v0.931.pdf

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