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

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