ArmVirtPkg: move QEMU based platforms to ArmVirtPL031FdtClientLib

ArmVirtPkg: move QEMU based platforms to ArmVirtPL031FdtClientLib

This moves QEMU based platforms to ArmVirtPL031FdtClientLib, so that we no
longer have to rely on VirtFdtDxe to execute first and set the PL031 base
address in a dynamic PCD.

The only driver which [transitively] depends on this PcdPL031RtcBase PCD is
EmbeddedPkg/RealTimeClockRuntimeDxe, so this conversion cannot affect any
other users and is thus safe.

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