IntelSiliconPkg/SpiFvbServiceSmm: Support Additional NVS region.

Platform may implement an additional NVS region following
Regular variable region and in this case SpiFvbService should include
both region size when calculating the total NVS region size.

The PcdFlashNvStorageAdditionalSize is for compatible with legacy
usages that should be deprecated. The new usage model should define
separate regions without implicit connections to UEFI Variable or
FTW regions.

Example NVS flash map for such legacy usage:
Note: PcdFlashNvStorageAdditionalSize is equal to platform
      PcdFlashFvNvStorageEventLogSize.

  ---------------
  |UEFI Variable|
  ---------------
  |EventLog     | <= this is Additional NVS region
  ---------------
  |FTW Working  |
  ---------------
  |FTW Spare    |
  ---------------

Cc: Ashraf Ali S <ashraf.ali.s@intel.com>
Cc: Isaac Oram <isaac.w.oram@intel.com>
Cc: Rangasai V Chaganty <rangasai.v.chaganty@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Michael Kubacki <michael.kubacki@microsoft.com>
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Isaac Oram <isaac.w.oram@intel.com>