bindings: PCI: artpec: correct pci binding example

 - Increase config size. When using a PCIe switch,
   the previous config size only had room for one device.
 - Add bus range. Inherited optional property.
 - Map downstream I/O to PCI address 0. We can map it to any
   address, but let's be consistent with other drivers.

Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Rob Herring <robh@kernel.org>