spapr_pci: Make find_phb()/find_dev() public

This makes find_phb()/find_dev() public and changed its names
to spapr_pci_find_phb()/spapr_pci_find_dev() as they are going to
be used from other parts of QEMU such as VFIO DDW (dynamic DMA window)
or VFIO PCI error injection or VFIO EEH handling - in all these
cases there are RTAS calls which are addressed to BUID+config_addr
in IEEE1275 format.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
