[POWERPC] Resolve the parent address of a PCI bus range

When parsing the OF "ranges" properties of PCI host busses to determine
the mapping of a PCI bus, we need to translate the "parent" address using
the prom_parse.c routines in order to obtain a CPU physical address.

This wasn't necessary while PCI busses were always at the root of the
device-tree but this is no longer the case on Cell where they can be
anywhere in the tree.

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