Add device location wiring to the pci bus.

This allows one to specify, for example, that if there's an igb card
in bus 12, slot 0, function 0, it should be assigned igb5. If there
isn't, or there's one in a different slot, normal numbering rules
apply (hinted units are skipped). Adding 'hint.igb.5.at="pci12:0:0"'
or 'hint.igb.5.at="pci0:12:0:0"' to /boot/device.hints will accomplish
this. The double quotes are important.

The kernel only accepts the strings (in shell notation):
pci$d:$b:$s:$f
and pci$b:$s:$f
where $d is the pci domain, $b is the pci bus number, $s is the slot
number and $f is the function number. A string compare is done with
the current device to avoid another string parser in the kernel. All
numbers are unsigned decimal without leading zeros.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D13546