MdeModulePkg: stop abusing EFI_EVENT for protocol notify registration

EfiCreateProtocolNotifyEvent() takes a (VOID**) for "Registration",
similarly to gBS->RegisterProtocolNotify(). We should pass the address of
an actual pointer-to-VOID, and not the address of an EFI_EVENT. EFI_EVENT
just happens to be specified as (VOID*), and has nothing to do with the
registration.

The same applies to gMmst->MmRegisterProtocolNotify().

"mFtwRegistration", "mFvRegistration", and "mFvbRegistration" are used for
nothing else.

This change is a no-op in practice; it's a semantic improvement.

Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Ray Ni <ray.ni@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Hao A Wu <hao.a.wu@intel.com>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>

MdeModulePkg/Universal/FaultTolerantWriteDxe/FaultTolerantWriteSmm.c

@@ -56,7 +56,7 @@ SPDX-License-Identifier: BSD-2-Clause-Patent
 #include "FaultTolerantWriteSmmCommon.h"
 #include <Protocol/MmEndOfDxe.h>
 
-EFI_EVENT                                 mFvbRegistration = NULL;
+VOID                                      *mFvbRegistration = NULL;
 EFI_FTW_DEVICE                            *mFtwDevice      = NULL;
 
 ///