ArmVirtPkg/FdtClientDxe: install DT as sysconfig table in protocol notify

Replace the dependency on PcdPureAcpiBoot with a Platform Has Device Tree
notification callback. Move the sysconfig table installation from the
entry point function to the callback.

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