MdeModulePkg/Core: Support standalone MM in FV2 protocol GetNextFile().
    
    REF:https://bugzilla.tianocore.org/show_bug.cgi?id=3028
    
    The FirmwareVolume2Protocol->GetNextFile() produced by DXE Core can be used
    to search for a file based on the value of *FileType input. However, this
    service will always return EFI_NOT_FOUND if the input FileType is set to
    EFI_FV_FILETYPE_MM_STANDALONE or EFI_FV_FILETYPE_MM_CORE_STANDALONE, Which
    means user can't use this service to search any standalone MM image in that
    FV.
    This patch update the FirmwareVolume2Protocol->GetNextFile() service to
    support searching standalone MM module.
    
    Signed-off-by: Siyuan Fu <siyuan.fu@intel.com>
    Cc: Dandan Bi <dandan.bi@intel.com>
    Cc: Liming Gao <gaoliming@byosoft.com.cn>
    Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>