arm64: PCI: Drop DT IRQ allocation from pcibios_alloc_irq()

With the introduction of struct pci_host_bridge.map_irq pointer it is
possible to assign IRQs for all devices originating from a PCI host bridge
at probe time; this is implemented through pci_assign_irq() that relies on
the struct pci_host_bridge.map_irq pointer to map IRQ for a given device.

The benefits this brings are twofold:

  - the IRQ for a device is assigned once at probe time
  - the IRQ assignment works also for hotplugged devices

With all DT based PCI host bridges converted to the struct
pci_host_bridge.{map/swizzle}_irq hooks mechanism the DT IRQ allocation in
ARM64 pcibios_alloc_irq() is now redundant and can be removed.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Will Deacon <will.deacon@arm.com>