Turns out that ye olde siba.c is /just/ the siba mips code (used by
the initial SENTRY5 port. However, I don't think it was ever
finished enough to be useful, and I do have this nagging feeling
that we'll eventually replace it with the bhnd code.
But, since bhnd(4) introduced siba.c too, we ended up with a
source file name clash, and that broke the SENTRY5 build.
It /looks/ like this is the only place siba.c / device siba is
used.
45d8d7918e