PCI: Move of_irq_parse_and_map_pci() declaration under OF_IRQ

Since commit 4670d610 ("PCI: Move OF-related PCI functions into
PCI core"), sparc:allmodconfig fails to build with the following error.

  pcie-cadence-host.c:(.text+0x4c4): undefined reference to `of_irq_parse_and_map_pci'
  pcie-cadence-host.c:(.text+0x4c8): undefined reference to `of_irq_parse_and_map_pci'

of_irq_parse_and_map_pci() is now only available if OF_IRQ is enabled.
Make its declaration and its dummy function dependent on OF_IRQ to solve
the problem.

Fixes: 4670d610 ("PCI: Move OF-related PCI functions into PCI core")
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>