pci: declare pciauto functions in header

The FSL PCI driver uses local prototypes for
pciauto_[pre|post]scan_setup_bridge(), this does not seem right,
so move them to the <pci.h> file.

Fixed a small extern declaration too, this is harmless but distracts
the view since all other prototypes are explicitly external.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>