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/EmbeddedPkg.dec

@@ -62,6 +62,12 @@
   ## Include/Guid/PlatformHasDeviceTree.h
   gEdkiiPlatformHasDeviceTreeGuid = { 0x7ebb920d, 0x1aaf, 0x46d9, { 0xb2, 0xaf, 0x54, 0x1e, 0x1d, 0xce, 0x14, 0x8b } }
 
+  # HII form set GUID for DtPlatformDxe driver
+  gDtPlatformFormSetGuid = { 0x2b7a240d, 0xd5ad, 0x4fd6, { 0xbe, 0x1c, 0xdf, 0xa4, 0x41, 0x5f, 0x55, 0x26 } }
+
+  # File GUID for default DTB image embedded in the firmware volume
+  gDtPlatformDefaultDtbFileGuid = { 0x25462cda, 0x221f, 0x47df, { 0xac, 0x1d, 0x25, 0x9c, 0xfa, 0xa4, 0xe3, 0x26 } }
+
 [Protocols.common]
   gHardwareInterruptProtocolGuid =  { 0x2890B3EA, 0x053D, 0x1643, { 0xAD, 0x0C, 0xD6, 0x48, 0x08, 0xDA, 0x3F, 0xF1 } }
   gEfiDebugSupportPeriodicCallbackProtocolGuid = { 0x9546e07c, 0x2cbb, 0x4c88, { 0x98, 0x6c, 0xcd, 0x34, 0x10, 0x86, 0xf0, 0x44 } }