[POWERPC] iSeries: eliminate pci_dn bussubno

xlate_iomm_address() really wants the ds_addr to pass to the HV, so store
that value (instead of the BAR number) when we allocate the device bars.
This is not a fast path, so we can look up the device_node property
there instead of using the bussubno field of the pci_dn.

The other user of iseries_ds_addr() was already scanning the device tree,
so looking up a property will not slow it down any more.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>