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.