ArmVirtPkg: move TimerDxe to FDT client library

ArmVirtPkg: move TimerDxe to FDT client library

Move to the new dedicated ArmVirtTimerFdtClientLib to populate the
various timer related PCDs at driver load time rather than relying on
VirtFdtDxe to do it. Since ArmPkg/TimerDxe is the only consumer of these
PCDs, which is the DXE driver ArmVirtTimerFdtClientLib is intended to
complement, 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>