This allows us to convert aucom to just another com attachment, and cleanup
some code in the com_arbus.c.
Additionally, we use a common com_cleanup routine rather than having a
zillion copies of it in the attachment points.
This has been tested on a number architectures, and it has been shown to get
close to comparable performance when COM_REGMAP is defined, and comparable
when it is not defined.
Approved by core@. Fixes PR port-evbmips/32362.
(coming in a the follow up commit for dbau1550 only), and is not yet complete.
It has serious problems, enough that it isn't yet usable, although the
functionality is all basically fleshed out. It is not enabled in any
default kernels at this point, so it should be benign. Hopefully the
bugs will soon be worked out and these caveats can be removed.
