PCI: rcar: Break out window size handling

Break out the hard coded window size code to allow dynamic setup. The
window size is still left at 1GiB but with this patch changing window size
is easy for testing.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Simon Horman <horms+renesas@verge.net.au>