ofw: add BUS_GET_DEVICE_PATH interface to openfirm/fdt, somewhat incomplete.

This add BUS_GET_DEVICE_PATH interface,
which shows device tree of openfirm/fdt.

In qemu-system-arm64 with "virt" machine with device-tree firmware,
% devctl getpath OFW cpu0

Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D37031