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.
attached to "obio") on the IQ80310 and IQ80321. It makes more sense
to do it this way for this type of system (the goal being to encapsulate
as much information about the board as possible into one file).
