OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Do not expose MMIO in SMM build

OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Do not expose MMIO in SMM build

In the SMM build, only an SMM driver is using the address range hence we
do not need to expose the flash MMIO range in EFI runtime mapping.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien.grall@linaro.org>
Cc: Justen Jordan L <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>