StandaloneMmPkg: make package .DSC file build again
    
    The StandaloneMmPkg .DSC file went out of sync with the changes
    applied to the package when I enabled this code on the Synquacer
    platform in edk2-platforms. So apply the necessary changes to make
    this package build in isolation.
    
    Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>
    Build-tested-by: Laszlo Ersek <lersek@redhat.com>
    Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>