soundwire: Add MIPI DisCo property helpers

MIPI Discovery And Configuration (DisCo) Specification for SoundWire
specifies properties to be implemented for SoundWire Masters and
Slaves. The DisCo spec doesn't mandate these properties. However,
SDW bus cannot work without knowing these values.

The helper functions read the Master and Slave properties.
Implementers of Master or Slave drivers can use any of the below
three mechanisms:
   a) Use these APIs here as .read_prop() callback for Master
      and Slave
   b) Implement own methods and set those as .read_prop(), but invoke
      APIs in this file for generic read and override the values with
      platform specific data
   c) Implement ones own methods which do not use anything provided
      here

Signed-off-by: Sanyog Kale <sanyog.r.kale@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>