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>