PCI: Add R-Car Gen2 internal PCI support

This adds internal PCI controller driver for R-Car Gen2 SoC.  There are
three PCI controllers available with only a single EHCI/OHCI device
built-in on each PCI bus.  This gives us three USB channels.  Channel 0 is
shared with the USBHS device, while channel 2 is shared with the USBSS.

The PCI controllers do not support I/O port space mapping, and it is not
needed here.

Signed-off-by: Valentine Barshak <valentine.barshak@cogentembedded.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>