Documentation: driver: add SoundWire BRA description

Documentation: driver: add SoundWire BRA description

The Bulk Register Access protocol was left as a TODO topic since
2018. It's time to document this protocol and the design of its Linux
support.

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.dev>
Signed-off-by: Bard Liao <yung-chuan.liao@linux.intel.com>
Reviewed-by: Péter Ujfalusi <peter.ujfalusi@linux.intel.com>
Reviewed-by: Liam Girdwood <liam.r.girdwood@intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Tested-by: shumingf@realtek.com
Link: https://lore.kernel.org/r/20250227140615.8147-2-yung-chuan.liao@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>