MdeModulePkg/NvmExpressPei: Avoid updating the module-level variable

This commit is out of the scope for BZ-1409. The commit will remove the
call of RegisterForShadow() at the entry point of the driver. By doing so,
the driver is now possible to be executed without being re-loaded into
permanent memory.

Thus, this commit will update the NvmExpressPei driver to avoid updating
the content of a global variable.

Cc: Jian J Wang <jian.j.wang@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>

MdeModulePkg/Bus/Pci/NvmExpressPei/NvmExpressPei.h

@@ -2,7 +2,7 @@
   The NvmExpressPei driver is used to manage non-volatile memory subsystem
   which follows NVM Express specification at PEI phase.
 
-  Copyright (c) 2018, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2018 - 2019, Intel Corporation. All rights reserved.<BR>
 
   This program and the accompanying materials
   are licensed and made available under the terms and conditions
@@ -147,13 +147,9 @@ struct _PEI_NVME_CONTROLLER_PRIVATE_DATA {
   CR (a, PEI_NVME_CONTROLLER_PRIVATE_DATA, EndOfPeiNotifyList, NVME_PEI_CONTROLLER_PRIVATE_DATA_SIGNATURE)
 
 
-/**
-  Initialize IOMMU.
-**/
-VOID
-IoMmuInit (
-  VOID
-  );
+//
+// Internal functions
+//
 
 /**
   Allocates pages that are suitable for an OperationBusMasterCommonBuffer or