ArmPlatformPkg/ArmVirtualizationPkg: introduce PCDs for device tree base address

ArmPlatformPkg/ArmVirtualizationPkg: introduce PCDs for device tree base address

Introduce gArmVirtualizationTokenSpaceGuid.PcdDeviceTreeBaseAddress and
PcdDeviceTreeInitialBaseAddress, which will be used by virtual machine ports
that discover the system configuration from a flattened device tree DTB image.

The latter is FixedPcd only, and should contain the initial offset of the DTB,
the former may be declared as dynamic, and updated at runtime if the DTB is
relocated before the DXE phase.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16135 6f19259b-4bc3-4df7-8a09-765794883524