PCI: generic: Select IRQ_DOMAIN

The generic PCI host controller calls of_irq_parse_and_map_pci() in its IRQ
fixup, but that function is only available when CONFIG_IRQ_DOMAIN is set:

  drivers/pci/built-in.o: In function `pci_host_common_probe':
  drivers/pci/host/pci-host-common.c:181: undefined reference to `of_irq_parse_and_map_pci'

There is no downside in enabling the domains here, so use a Kconfig
select statement to ensure it's always available to this driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>