EmbeddedPkg: introduce EDKII Platform Has Device Tree GUID

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>