PCI: xgene: Move to struct pci_host_bridge IRQ mapping functions

struct pci_host_bridge gained hooks to map/swizzle IRQs, so that the IRQ
mapping can be done automatically by PCI core code through the
pci_assign_irq() function instead of resorting to arch-specific
implementation callbacks to carry out the same task which force PCI host
bridge drivers implementation to implement per-arch kludges to carry out a
task that is inherently architecture agnostic.

Add map/swizzle IRQs hooks to the xgene PCI host driver to move the IRQ
allocation into core code and stop relying on arch-specific callbacks.

Tested-by: Khuong Dinh <kdinh@apm.com>	# with e1000e
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Tanmay Inamdar <tinamdar@apm.com>