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

[takawata]

Summary

This add BUS_GET_DEVICE_PATH allow device tree of openfirm/fdt.

Test Plan

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

[andrew]

It might be better to name the selector "OFW" to be consistant with the directory name, ofwdump, etc.

sys/arm/arm/gic.c

1215

This should be in gic_fdt.c

sys/dev/ofw/ofw_bus_subr.c

113

Extra space before strcmp and "OF" should be BUS_LOCATOR_OF and defined in sys/bus.h

120

return (bus_generic_get_device_path(...));