MdeModulePkg Variable: Align TPL level for (Smm)EndOfDxe callback

VariableRuntimeDxe will have OnEndOfDxe() callback function at
TPL_NOTIFY level on EndOfDxe event when DXE variable solution is
used.
  Status = gBS->CreateEventEx (
                  EVT_NOTIFY_SIGNAL,
                  TPL_NOTIFY,
                  OnEndOfDxe,
                  NULL,
                  &gEfiEndOfDxeEventGroupGuid,
                  &EndOfDxeEvent
                  );

VariableSmm will have SmmEndOfDxeCallback() callback function at
TPL_CALLBACK level on SmmEndOfDxe event when SMM variable solution
is used.
SmmIplGuidedEventNotify()  -  PiSmmIpl.c TPL_CALLBACK on EndOfDxe
->
SmmEndOfDxeHandler()  -  PiSmmCore.c install SmmEndOfDxe protocol
->
SmmEndOfDxeCallback() - VariableSmm.c

The TPL level for (Smm)EndOfDxe callback between VariableRuntimeDxe
and VariableSmm is inconsistent, it will make the unified platform
code could not make sure its TPL_NOTIFY EndOfDxe callback function
(to use variable lock/check) executed before (Smm)EndOfDxe callback
function in variable driver. The variable lock/check will start to
protect after (Smm)EndOfDxe callback function in variable driver is
executed.

This patch is to algin the TPL level to TPL_CALLBACK for (Smm)EndOfDxe
callback between VariableRuntimeDxe and VariableSmm.

Cc: Chasel Chiu <chasel.chiu@intel.com>
Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Chasel Chiu <chasel.chiu@intel.com>

MdeModulePkg/Universal/Variable/RuntimeDxe/VariableDxe.c

@@ -3,7 +3,7 @@
   and volatile storage space and install variable architecture protocol.
 
 Copyright (C) 2013, Red Hat, Inc.
-Copyright (c) 2006 - 2017, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2006 - 2018, Intel Corporation. All rights reserved.<BR>
 (C) Copyright 2015 Hewlett Packard Enterprise Development LP<BR>
 This program and the accompanying materials
 are licensed and made available under the terms and conditions of the BSD License
@@ -547,7 +547,7 @@ VariableServiceInitialize (
   //
   Status = gBS->CreateEventEx (
                   EVT_NOTIFY_SIGNAL,
-                  TPL_NOTIFY,
+                  TPL_CALLBACK,
                   OnEndOfDxe,
                   NULL,
                   &gEfiEndOfDxeEventGroupGuid,