bus: Introduce the bus interface get_device_path

This returns the full path of a the child device requested. Since
there's different ways to recon the entire path, include a 'locator'
method. The default 'FreeBSD' method uses a filesystem-like path name
with each device to the root node separated by /. Other locators will be
UEFI, ACPI and fdt, though others are possible in the future. Make the
locator a string to allow maximum flexibility.

Sponsored by: Netflix
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D32744