pci: rmobile: Add RCar Gen2 PCIe controller driver

Add driver for the Renesas RCar PCIe controller present on Gen2 SoCs.
The PCIe on Gen2 is used both to connect external PCIe peripherals as
well as access the on-SoC USB EHCI controller.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>