FmpDevicePkg: Add FmpDependencyDevice library class and NULL instance

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2696

This library provides firmware device specific services to support
saving dependency to firmware device and getting dependency from
firmware device.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Signed-off-by: Wei6 Xu <wei6.xu@intel.com>
Reviewed-by: Sean Brogan <sean.brogan@microsoft.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

FmpDevicePkg/Library/FmpDependencyDeviceLibNull/FmpDependencyDeviceLib.c

@@ -0,0 +1,50 @@
+/** @file
+  Null instance of FmpDependencyDeviceLib.
+
+  Copyright (c) 2020, Intel Corporation. All rights reserved.<BR>
+
+  SPDX-License-Identifier: BSD-2-Clause-Patent
+
+**/
+#include <PiDxe.h>
+#include <Library/FmpDependencyDeviceLib.h>
+
+/**
+  Save dependency to Fmp device.
+
+  @param[in]  Depex       Fmp dependency.
+  @param[in]  DepexSize   Size, in bytes, of the Fmp dependency.
+
+  @retval  EFI_SUCCESS       Save Fmp dependency succeeds.
+  @retval  EFI_UNSUPPORTED   Save Fmp dependency is not supported.
+  @retval  Others            Save Fmp dependency fails.
+
+**/
+EFI_STATUS
+EFIAPI
+SaveFmpDependency (
+  IN EFI_FIRMWARE_IMAGE_DEP  *Depex,
+  IN UINT32                  DepexSize
+  )
+{
+  return EFI_UNSUPPORTED;
+}
+
+/**
+  Get dependency from the Fmp device.
+  This caller is responsible for freeing the dependency buffer.
+
+  @param[out]  DepexSize   Size, in bytes, of the dependency.
+
+  @retval  The pointer to dependency.
+  @retval  NULL
+
+**/
+EFI_FIRMWARE_IMAGE_DEP*
+EFIAPI
+GetFmpDependency (
+  OUT UINT32  *DepexSize
+  )
+{
+  return NULL;
+}