ArmVirtPkg/ArmVirtPL031FdtClientLib: unconditionally disable DT node

Disable the PL031 RTC DT node unconditionally rather than only when
the DT will be exposed to the OS. This allows us to defer the decision
whether to expose it to the OS to a later time without creating an
additional dependency on the FDT client code by the RTC driver.

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>