Add device tree probing support to the 'pci-rcar-gen2' driver.

[Sergei: numerous fixes/cleanups/additions]
[bhelgaas: whitespace fix]
Signed-off-by: NBen Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: NSergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Acked-by: NSimon Horman <horms+renesas@verge.net.au>
Acked-by: NArnd Bergmann <arnd@arndb.de>