MdePkg UefiLib: Make the event empty function public

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=298

Then this event empty function can be used to remove
the duplication in drivers and other libraries.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Michael Kinney <michael.d.kinney@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdePkg/Include/Library/UefiLib.h

@@ -12,7 +12,7 @@
   of size reduction when compiler optimization is disabled. If MDEPKG_NDEBUG is
   defined, then debug and assert related macros wrapped by it are the NULL implementations.
 
-Copyright (c) 2006 - 2012, Intel Corporation. All rights reserved.<BR>
+Copyright (c) 2006 - 2017, Intel Corporation. All rights reserved.<BR>
 This program and the accompanying materials are licensed and made available under 
 the terms and conditions of the BSD License that accompanies this distribution.  
 The full text of the license may be found at
@@ -241,6 +241,22 @@ EfiEventGroupSignal (
   IN CONST EFI_GUID *EventGroup
   );
 
+/**
+  An empty function that can be used as NotifyFunction parameter of
+  CreateEvent() or CreateEventEx().
+
+  @param Event              Event whose notification function is being invoked.
+  @param Context            The pointer to the notification function's context,
+                            which is implementation-dependent.
+
+**/
+VOID
+EFIAPI
+EfiEventEmptyFunction (
+  IN EFI_EVENT              Event,
+  IN VOID                   *Context
+  );
+
 /** 
   Returns the current TPL.