PCI: Look up IRQ domain by fwnode_handle

If pci_host_bridge_msi_domain() can't find an IRQ domain through the OF
tree, try to look it up directly through the fwnode_handle.

[bhelgaas: changelog]
Signed-off-by: Jake Oshins <jakeo@microsoft.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>