pseries: Export find_phb() utility function for PCI code

The pseries PCI code makes use of an internal find_dev() function which
locates a PCIDevice * given a (platform specific) bus ID and device
address.  Internally this needs to first locate the host bridge on which
the device resides based on the bus ID.  This patch exposes that host
bridge lookup as a separate function, which we will need later in the MSI
and VFIO code.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[agraf: drop trace.h inclusion]
Signed-off-by: Alexander Graf <agraf@suse.de>
