OvmfPkg/BaseMemEncryptTdxLib: Add TDX helper library

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

Add Intel Tdx helper library. The library provides the routines to:
-  set or clear Shared bit for a given memory region.
-  query whether TDX is enabled.

Please be noted, there a lot of duplicated codes for Page Table
operations. These codes should be moved to a common library
(PageTablesLib) so that it is more friendly for review and maintain.
There is a new feature requirement
https://bugzilla.tianocore.org/show_bug.cgi?id=847 which is to
implement the library. After the lib is introduced, BaseMemEncryptTdxLib
will be refactored.

Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Jordan Justen <jordan.l.justen@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: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>

OvmfPkg/Library/BaseMemEncryptTdxLib/BaseMemEncryptTdxLibNull.inf

@@ -0,0 +1,35 @@
+## @file
+#  Library for Memory Encryption
+#
+# Copyright (c) 2020, Intel Corporation. All rights reserved.<BR>
+# Copyright (c) 2017 Advanced Micro Devices. All rights reserved.<BR>
+#
+#  SPDX-License-Identifier: BSD-2-Clause-Patent
+#
+#
+##
+
+[Defines]
+  INF_VERSION                    = 1.25
+  BASE_NAME                      = MemEncryptTdxLibNull
+  FILE_GUID                      = 3C69C4CA-DE46-44D7-8AA5-6EE51A4E3EA7
+  MODULE_TYPE                    = BASE
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = MemEncryptTdxLib|PEIM DXE_DRIVER DXE_RUNTIME_DRIVER DXE_SMM_DRIVER UEFI_DRIVER
+
+#
+# The following information is for reference only and not required by the build
+# tools.
+#
+# VALID_ARCHITECTURES           = X64 IA32
+#
+
+[Packages]
+  MdePkg/MdePkg.dec
+  OvmfPkg/OvmfPkg.dec
+
+[Sources]
+  BaseMemoryEncryptionNull.c
+
+[LibraryClasses]
+  BaseLib