ArmVirtualizationPkg: allow patchable PCD for device tree base address

ArmVirtualizationPkg: allow patchable PCD for device tree base address

To allow a runtime self relocating PrePi instance to discover the base
address of the device tree at runtime, allow the use of a patchable PCD
for gArmVirtualizationTokenSpaceGuid.PcdDeviceTreeInitialBaseAddress.
We will not be using the build time patch tool in this case, but using
a patchable PCD will make the build system aware that its value is not
a compile time constant.

Contributed-under: TianoCore Contribution Agreement 1.0
Reviewed-by: Olivier Martin <olivier.martin@arm.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16957 6f19259b-4bc3-4df7-8a09-765794883524