OvmfPkg/TdxMailboxLib: Add NULL instance of TdxMailboxLib

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=4172

TdxMailboxLib is designed only for TDX guest which arch is X64. This
patch set the VALID_ARCHITECTURES of TdxMailboxLib as X64.

Because in the following patches TdxMailboxLib will be included in
PlatformInitLib. While PlatformInitLib is imported by some X64 platforms
(for example AmdSevX64.dsc). So we need a NULL instance of TdxMailboxLib
which VALID_ARCHITECTURES is X64 as well. Based on this consideration
we design TdxMailboxLibNull.

Cc: Erdem Aktas <erdemaktas@google.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Cc: James Bottomley <jejb@linux.ibm.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Min Xu <min.m.xu@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>

OvmfPkg/Library/TdxMailboxLib/TdxMailboxLib.inf

@@ -1,6 +1,6 @@
 #/** @file
 #
-#  TBD
+#  TdxMailbox Library
 #
 #  Copyright (c) 2018, Intel Corporation. All rights reserved.<BR>
 #  Copyright (c) 2008, Apple Inc. All rights reserved.<BR>
@@ -19,12 +19,9 @@
   LIBRARY_CLASS                  = TdxMailboxLib
 
 #
-#  VALID_ARCHITECTURES           = X64 IA32
+#  VALID_ARCHITECTURES           = X64
 #
 
-[Sources.IA32]
-  TdxMailboxNull.c
-
 [Sources.X64]
   TdxMailbox.c