arm64: PCI: Preserve firmware configuration when desired

If we must preserve the firmware resource assignments, claim the existing
resources rather than reassigning everything.

Link: https://lore.kernel.org/r/20190615002359.29577-4-benh@kernel.crashing.org


Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>