This is almost a complete rewrite from the old hardware_gen.py.
It separates the 'parse DT' stage from the 'generate output'
devices more strictly, and is hopefully easier to understand and
easier to extend.
We also no longer generate the 'devices' list (in YAML)
or the dev_p_regs array (in C), as the kernel will implicitly
expose all non-RAM untypeds as devices.
75f2c54b42