PCI: Add pcibios_bus_add_device() weak function

This adds weak function pcibios_bus_add_device() for arch dependent
code could do proper setup. For example, powerpc could setup EEH
related resources for SRIOV VFs.

Signed-off-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Reviewed-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>