ArmVirtPkg: implement ArmVirtTimerFdtClientLib

ArmVirtPkg: implement ArmVirtTimerFdtClientLib

This implements a library ArmVirtTimerFdtClientLib which is intended to
be incorporated into TimerDxe via NULL library class resolution. This
allows us to make TimerDxe depend on the FDT client protocol, and
discover the timer interrupts from the device tree directly rather than
relying on VirtFdtDxe to set the dynamic PCDs.

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