OvmfPkg: Add TdxMailboxLibNull in some platform dsc

OvmfPkg: Add TdxMailboxLibNull in some platform dsc

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

In the following patches TdxMailboxLib will be included in
PlatformInitLib. While PlatformInitLib is imported by some IA32/X64
platforms (for example AmdSevX64.dsc). So TdxMailboxLibNull is added in
those platforms which don't support TDX feature.

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>