powerpc/PCI: factor out pcibios_setup()

The PCI core provides a generic pcibios_setup() routine.  Drop this
architecture-specific version in favor of that.

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Myron Stowe <myron.stowe@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>