[POWERPC] celleb: Fix support for multiple PCI domains

Celleb has multiple PCI host bridges (phbs).  Previous boot logic gives
non-overlapped bus IDs between PCI host bridges so you can identify
PHB by bus ID.  But newer boot logic gives same bus ID between PHBs (it
gives bus ID 0 as root bus.) So we have to set 'phb->buid' as
non-zero.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Paul Mackerras <paulus@samba.org>