pci: move pciauto_config_init() to pci.h

Fixing build regressions for the Integrator I get find that a few
boards try to work around the missing declaration of
pciauto_config_init() by declaring it in the local scope. This
does not make sense when the sibling functions are in <pci.h>
so move the function to the header, ridding the build error
in the Integrator and getting rid of the local declarations
here and there.

Reported-by: Wolfgang Denk <wd@denx.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>