EmbeddedPkg: add DT platform driver to select between DT and ACPI

As a follow up to the changes proposed by Laszlo to make ACPI and DT
mutually exclusive on ArmVirtQemu, this patch proposes a DT platform
DXE driver that either installs the NULL protocol PlatformHasAcpiGuid,
or installs the FV embedded DTB binary as a configuration table under
the appropriate GUID, depending on a preference setting recorded as
a UEFI variable, and configurable via a HII screen.

The DTB binary can be embedded in the firmware image by adding the
following to the platform .fdf file:

  FILE FREEFORM = 25462CDA-221F-47DF-AC1D-259CFAA4E326 {
    SECTION RAW = SomePkg/path/to/foo.dtb
  }

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

EmbeddedPkg/Drivers/DtPlatformDxe/DtPlatformDxe.h

@@ -0,0 +1,31 @@
+/** @file
+*
+*  Copyright (c) 2017, Linaro 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.
+*
+**/
+
+#ifndef __DT_PLATFORM_DXE_H__
+#define __DT_PLATFORM_DXE_H__
+
+#include <Guid/HiiPlatformSetupFormset.h>
+#include <Guid/DtPlatformFormSet.h>
+
+#define DT_ACPI_SELECT_DT       0x0
+#define DT_ACPI_SELECT_ACPI     0x1
+
+#define DT_ACPI_VARIABLE_NAME   L"DtAcpiPref"
+
+typedef struct {
+  UINT8         Pref;
+  UINT8         Reserved[3];
+} DT_ACPI_VARSTORE_DATA;
+
+#endif