ArmPkg: MTL Library interface and Null library implementation

Upcoming new component ArmPkg/Drivers/ArmScmiDxe is dependent on
platform specific ArmMtlLib library implementation, however in order
to be able to build the ArmScmiDxe component outside of the context of a
particular platform, this change adds Null implementation of the
ArmMtlLib along with ARM MTL library header.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Girish Pathak <girish.pathak@arm.com>
Signed-off-by: Evan Lloyd <evan.lloyd@arm.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>

ArmPkg/Library/ArmMtlNullLib/ArmMtlNullLib.inf

@@ -0,0 +1,26 @@
+#/** @file
+#  Copyright (c) 2017-2018, Arm Limited. All rights reserved.
+#
+#  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.
+#**/
+
+[Defines]
+  INF_VERSION                    = 0x00010019
+  BASE_NAME                      = ArmMtlNullLib
+  FILE_GUID                      = 05810525-FDEC-4006-9F1F-37609B3675FA
+  MODULE_TYPE                    = BASE
+  VERSION_STRING                 = 1.0
+  LIBRARY_CLASS                  = ArmMtlLib
+
+[Sources.common]
+  ArmMtlNullLib.c
+
+[Packages]
+  ArmPkg/ArmPkg.dec
+  MdePkg/MdePkg.dec