1) Rather than build an array of the pci->intr mappings, build a proplib
tree.
2) add a SIMPLEQ of device properties to the pci_chipset_tag to hold the
proplib trees of each bus.
3) Move the interrupt routing code from pci_conf_interrupt to pci_map_intr()
4) Deal properly with non-native PCI bridge chips that are not recognized
by the prep firmware, and therefore are not noted in the residual data.
The major win of this restructure is #4. Hopefully I haven't broken
anything. Tested on a 7248-100, 7043-140 and 7024-E20.
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