[SPARC64]: More SUN4V PCI work.

Get bus range from child of PCI controller root nexus.
This is actually a hack, but the PCI-E bridge sitting
at the top of the PCI tree responds to PCI config cycles
for every device number, so best to just ignore it for now.

Preliminary PCI irq routing, needs lots of work.

Signed-off-by: David S. Miller <davem@davemloft.net>