Updates to MPC85XX PCI driver

[Nathan Whitehorn]

I've been going through our tree trying to remove code duplication 
between AIM and Book-E, as people attentive to the commit logs might 
have seen. At the same time, I'm also trying to deduplicate the FDT and 
OFW code, since they are implementing many of the same interfaces and 
specifications. As part of that, I've adapted the MPC85XX PCI driver to 
use the generic ofw_pci code, which gives a quite substantial reduction 
in the driver and also substantially improves its robustness to 
variations on the device tree setup:
http://people.freebsd.org/~nwhitehorn/pci_mpc85xx.diff

I can currently only test this patch in QEMU, where it works fine, and 
there should be no functional changes at all, but tests on real hardware 
would be appreciated. If I don't hear anything, I'll commit this at the 
end of the week.
-Nathan