PCI: rockchip: Factor out rockchip_pcie_get_phys()

We plan to introduce per-lane PHYs, so factor out rockchip_pcie_get_phys()
to make it easier in the future.  No functional change intended.

Tested-by: Jeffy Chen <jeffy.chen@rock-chips.com>
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>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>