PCI: rcar: Allow DT to override default window settings

If the DTB specifies dma-ranges, use those values.  Otherwise, default to
the values that were previously hardcoded into the driver.

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