PCI: Factor DT-specific pci_bus_find_domain_nr() code out

pci_bus_find_domain_nr() retrieves the host bridge domain number in a
DT-specific way.  Rename it to of_pci_bus_find_domain_nr() to reflect that,
so we can add a corresponding function for ACPI.

[bhelgaas: changelog]
Signed-off-by: Tomasz Nowicki <tn@semihalf.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>