PCI: Make resetting secondary bus logic common

Commit d92a208d

 ("powerpc/pci: Mask linkDown on resetting PCI bus")
implemented same logic (resetting PCI secondary bus by bridge's config
register PCI_BRIDGE_CTL_BUS_RESET) in PCI core and arch-dependent code.  To
avoid the duplication, move the logic to pci_reset_secondary_bus().

That commit did not declare the pcibios_reset_secondary_bus() interface in
linux/include/pci.h.  Add the declaration.

No functional change.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>