EmbeddedPkg: introduce EDKII Platform Has Device Tree GUID

The presence of this GUID in the PPI database, and/or in the DXE protocol
database (as dictated by the platform's needs in these firmware phases)
implies that the platform provides the operating system with a Device
Tree-based hardware description. This is not necessarily exclusive with
other types of hardware description (for example, an ACPI-based one).

A platform PEIM and/or DXE driver is supposed to produce a single instance
of the PPI and/or protocol (with NULL contents), if appropriate. The
decision to produce the PPI and/or protocol is platform specific; for
example, in the DXE phase, it could depend on an HII checkbox / underlying
non-volatile UEFI variable.

In the DXE phase, the protocol is meant to be consumed by the platform
driver that
- owns the Device Tree description of the hardware, and
- is responsible for installing it as a system configuration table.

Said FDT-owner driver can wait for the protocol via DEPEX or protocol
notify.

Because this GUID is not standard, it is prefixed with EDKII / Edkii, as
seen elsewhere (for example in MdeModulePkg and SecurityPkg).

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

EmbeddedPkg/EmbeddedPkg.dec

@@ -59,6 +59,9 @@
   ## Include/Guid/PlatformHasAcpi.h
   gEdkiiPlatformHasAcpiGuid = { 0xf0966b41, 0xc23f, 0x41b9, { 0x96, 0x04, 0x0f, 0xf7, 0xe1, 0x11, 0x96, 0x5a } }
 
+  ## Include/Guid/PlatformHasDeviceTree.h
+  gEdkiiPlatformHasDeviceTreeGuid = { 0x7ebb920d, 0x1aaf, 0x46d9, { 0xb2, 0xaf, 0x54, 0x1e, 0x1d, 0xce, 0x14, 0x8b } }
+
 [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 } }