PCI: Add missing DT binding for "linux,pci-domain" property

41e5c0f81d3e ("of/pci: Add pci_get_new_domain_nr() and
of_get_pci_domain_nr()") added parsing of the "linux,pci-domain" property,
but didn't add the binding documentation.

Since this property will be supported by a number of host bridge drivers,
add it to the common PCI binding doc.

Fixes: 41e5c0f81d3e ("of/pci: Add pci_get_new_domain_nr() and of_get_pci_domain_nr()")
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Liviu Dudau <Liviu.Dudau@arm.com>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/pci/pci.txt

@@ -7,3 +7,14 @@ And for the interrupt mapping part:
 
 Open Firmware Recommended Practice: Interrupt Mapping
 http://www.openfirmware.org/1275/practice/imap/imap0_9d.pdf
+
+Additionally to the properties specified in the above standards a host bridge
+driver implementation may support the following properties:
+
+- linux,pci-domain:
+   If present this property assigns a fixed PCI domain number to a host bridge,
+   otherwise an unstable (across boots) unique number will be assigned.
+   It is required to either not set this property at all or set it for all
+   host bridges in the system, otherwise potentially conflicting domain numbers
+   may be assigned to root buses behind different host bridges.  The domain
+   number for each host bridge in the system must be unique.