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: NShawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NBrian Norris <briannorris@chromium.org>