Documentation: Add SoundWire summary

SoundWire is a new Linux bus which implements a new MIPI bus protocol
'SoundWire'. The summary of SoundWire bus and API is documented in the
'summary' file.

Signed-off-by: Sanyog Kale <sanyog.r.kale@intel.com>
Signed-off-by: Hardik T Shah <hardik.t.shah@intel.com>
Reviewed-by: Philippe Ombredanne <pombredanne@nexb.com>
Acked-By: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Reviewed-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>