PCI: rockchip: Split out rockchip_cfg_atu()

Split out a new function, rockchip_cfg_atu(), in order to re-configure the
ATU when missing these information after wakeup from S3.

[bhelgaas: add "dev" temporary, return 0 when known]
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Brian Norris <briannorris@chromium.org>