ArmVirtPkg/FdtClientDxe: install DT configuration table at ReadyToBoot

Defer FDT configuration table installation until ReadyToBoot is signaled.
This allows any driver to make modifications in the mean time, and will
also allow us to defer the decision of whether to install it in the first
place to later on in the boot.

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