PCI: rockchip: Add Rockchip PCIe controller support

Add support for the Rockchip PCIe controller found on RK3399 SoC platform.

[bhelgaas: fold in Brian's rockchip_pcie_client_irq_handler() OR fix, other
fixes and cleanups from Guenter Roeck <linux@roeck-us.net> and me,
uninitialized variable fix from Arnd Bergmann <arnd@arndb.de>]
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>