x86: fix section mismatch in pci_scan_bus

Fix following section mismatch warning:
WARNING: vmlinux.o(.text+0x275616): Section mismatch in reference from the function pci_scan_bus() to the function .devinit.text:pci_scan_bus_parented()

The warning was seen with a CONFIG_DEBUG_SECTION_MISMATCH=y build.
The inline function pci_scan_bus refer to functions annotated
__devinit - so annotate it __devinit too.
This revealed a few x86 specific functions that were only
used from __init or __devinit context.
So annotate these __devinit and the warning was killed.

The added include in pci.h was not strictly required but
added to avoid being dependent on indirect includes.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Jesse Barnes <jbarnes@hobbes.lan>