PCI: pull pcibios_setup() up into core

Currently, all of the architectures implement their own pcibios_setup()
routine.  Most of the implementations do nothing so this patch introduces
a generic (__weak) routine in the core that can be used by all
architectures as a default.  If necessary, it can be overridden by
architecture-specific code.

Signed-off-by: Myron Stowe <myron.stowe@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>