bus: Add ACPI locator support

Add support for printing ACPI paths. This is a bit of a degenerate case
for this interface since it's always just the device handle if the
device has one. But it is illustrtive of how to do this for a few nodes
in the tree.

Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D32748