sim: sim-core: pass down cpu to hw accesses when available

sim: sim-core: pass down cpu to hw accesses when available

The bfin port has been using the device callback largely so it could be
passed the cpu when available.  Add this logic to the common core code
so all ports get access to the active cpu.

The semantics of these buffer functions are changed slightly in that
errors halt the engine synchronously rather than returning the length
to the caller.  We'll probably adjust this in a follow up commit.

The bfin code isn't updated just yet as it has a bit more logic in the
device layer that needs to be unwound at which point we can delete it
entirely.