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>