ac97: introduce first AC'97 decoder implementation (bits, slots, binary)

ac97: introduce first AC'97 decoder implementation (bits, slots, binary)

Introduce an "audio and modem control for PC systems" protocol decoder
(referred to as AC'97).

This implementation extracts bits and identifies frames, and annotates
the slots of a frame with mere integer values. Bit fields get decoded
depending on the slot numbers. Bit patterns in audio/modem data slots
can get exported as binary streams.

Some TODO items remain. Register access (read/write) gets annotated, but
neither gets interpreted nor affects the decoding of subsequent frames.
The RESET# line status does not get evaluated.