[POWERPC] Don't get PCI IRQ from OF for devices with no IRQ

This patch adds checking of the PCI_INTERRUPT_PIN register before
using standard OF parsing to retreive PCI interrupts. The reason is
that some PCI devices may have no PCI interrupt, though they may have
interrupts attached via other means. In this case, we shall not use
irq->pdev, but device-specific code can later retreive those interrupts
instead.

Without that patch, Maple and derivatives don't get the right interrupt
for the second IDE channel as the linux IDE code fallsback to the PCI
irq instead of trying to use the legacy ones for the on-board controller
(which has no PCI_INTERRUPT_PIN). Having no PCI IRQ assign to it (as it
doesn't request any) fixes it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>