OvmfPkg: Add PlatformHasIoMmuLib

Add the shorter-term library instance outlined in the previous patch to
OvmfPkg, so that we can imbue PciHostBridgeDxe with a protocol dependency
on gEdkiiIoMmuProtocolGuid OR gIoMmuAbsentProtocolGuid.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

OvmfPkg/Library/PlatformHasIoMmuLib/PlatformHasIoMmuLib.c

@@ -0,0 +1,33 @@
+/** @file
+  A hook-in library for MdeModulePkg/Bus/Pci/PciHostBridgeDxe.
+
+  Plugging this library instance into PciHostBridgeDxe makes
+  PciHostBridgeDxe depend on the platform's dynamic decision whether
+  to provide IOMMU implementation (usually through IoMmuDxe driver).
+
+  Copyright (C) 2017, Red Hat, Inc.
+  Copyright (C) 2017, AMD, Inc.
+
+  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 <Base.h>
+
+RETURN_STATUS
+EFIAPI
+PlatformHasIoMmuInitialize (
+  VOID
+  )
+{
+  //
+  // Do nothing, just imbue PciHostBridgeDxe with a protocol dependency on
+  // gIoMmuAbsentProtocolGuid OR gEdkiiIoMmuProtocolGuid.
+  //
+  return RETURN_SUCCESS;
+}