MdePkg: add MM_STANDALONE implementation of MmServicesTableLib

The newly introduced generic MmServicesTableLib can be implemented
for traditional SMM (as we did already), but MM_STANDALONE drivers
require an implementation that is based on standalone MM.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Liming Gao <liming.gao@intel.com>

MdePkg/Library/StandaloneMmServicesTableLib/StandaloneMmServicesTableLib.c

@@ -0,0 +1,41 @@
+/** @file
+  MM Services Table Library.
+
+  Copyright (c) 2009 - 2018, Intel Corporation. All rights reserved.<BR>
+  Copyright (c) 2018, Linaro, Ltd. All rights reserved.<BR>
+  This program and the accompanying materials
+  are licensed and made available under the terms and conditions of the BSD License
+  which accompanies this distribution.  The full text of the license may be found at
+  http://opensource.org/licenses/bsd-license.php.
+
+  THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS,
+  WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
+
+**/
+
+#include <PiMm.h>
+#include <Library/MmServicesTableLib.h>
+#include <Library/DebugLib.h>
+
+EFI_MM_SYSTEM_TABLE   *gMmst             = NULL;
+
+/**
+  The constructor function caches the pointer of the MM Services Table.
+
+  @param  ImageHandle   The firmware allocated handle for the EFI image.
+  @param  SystemTable   A pointer to the EFI System Table.
+
+  @retval EFI_SUCCESS   The constructor always returns EFI_SUCCESS.
+
+**/
+EFI_STATUS
+EFIAPI
+StandaloneMmServicesTableLibConstructor (
+  IN EFI_HANDLE           ImageHandle,
+  IN EFI_MM_SYSTEM_TABLE  *MmSystemTable
+  )
+{
+  gMmst = MmSystemTable;
+  ASSERT (gMmst != NULL);
+  return EFI_SUCCESS;
+}