PCI: Make pci_get_new_domain_nr() static

The only user of pci_get_new_domain_nr() is of_pci_bus_find_domain_nr().
Since they are defined in the same file, pci_get_new_domain_nr() can be
made static, which also simplifies preprocessor conditionals.

No functional change intended.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>