[RFC] soundwire: add runtime BPT support

[bardliao]

The driver may want to use BPT stream to download the firmware while an audio stream is running. This series calculates the available columns in a SoundWire frame and use them for the BPT stream. The streams are on on a first-come, first-served basis but will reserve 2 columns for the audio stream if possible. That should apply to most cases.