MdeModulePkg/FaultTolerantWriteDxe: factor out boot service accesses

MdeModulePkg/FaultTolerantWriteDxe: factor out boot service accesses

In preparation of providing a standalone MM based FTW driver, move
the existing SMM driver to the new MM services table, and factor out
some pieces that are specific to the traditional driver, mainly
related to the use of UEFI boot services, which are not accessible
to standalone MM drivers.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>