PCI: rcar: Set root bus nr to that provided in DT

On ARM64, setting the root bus number to -1 causes probe failure.
Moreover, we should use the bus number specified in the DT as we could have
multiple PCIe controllers with different bus ranges.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>