powerpc: Fix IRQ assignment for some PCIe devices

Currently, some PCIe devices on POWER6 machines do not get interrupts
assigned correctly.  The problem is that OF doesn't create an
"interrupt" property for them.  The fix is for of_irq_map_pci to fall
back to using the value in the PCI interrupt-pin register in config
space, as we do when there is no OF device-tree node for the device.

I have verified that this works fine with a pair of Squib-E SAS
adapter on a P6-570.

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