PCI: Add fwnode_handle to x86 pci_sysdata

Add an fwnode_handle to the x86 struct pci_sysdata, which will be used to
locate an IRQ domain associated with a root PCI bus.

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