ArmVirtPkg/ArmVirtPsciResetSystemLib: move to FDT client protocol

ArmVirtPkg/ArmVirtPsciResetSystemLib: move to FDT client protocol

Instead of relying on VirtFdtDxe to detect the PSCI method, move our
EfiResetSystemLib to the FDT client protocol to interrogate the device
tree directly.

Since this library is only consumed by EmbeddedPkg/ResetRuntimeDxe, and
considering that the PCD is no longer set, and even removed completely in a
subsequent patch, this conversion is guaranteed to be safe.

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