PCI: rockchip: Move configuration accesses into rockchip_pcie_cfg_atu()

Configuration accesses is also part of ATU settings, so let's keep all of
them inside rockchip_pcie_cfg_atu().

Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>