OF/PCI: Update of_irq_parse_and_map_pci() comment

With the introduction of struct pci_host_bridge.(*map_irq) function pointer
add another direct usage of of_irq_parse_and_map_pci().

Update the function comment to reflect this behaviour.

Signed-off-by: Matthew Minter <matt@masarand.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>