PCI: rcar: Drop gen2 dummy I/O port region

Previously we added a dummy I/O port region even though the R-Car
controller doesn't support PCI port I/O.  This resulted in bogus root bus
resources like this:

  pci_bus 0000:00: root bus resource [io  0xee080000-0xee0810ff]
  pci_bus 0000:00: root bus resource [mem 0xee080000-0xee0810ff]

Drop the unused dummy I/O port region and set struct hw_pci.io_optional so
the ARM PCI code doesn't add a default one for us.

Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>