[RESEND] ASoC: SOF: Intel: add build support for SoundWire

[Pierre-Louis Bossart]

> Can we enable SDW/SOF also for the 5.8 stable kernel or some vital pieces are
> missing in 5.8?
> 
> If I am not wrong, it's a 5.9 patch, right?

I just tried with v5.8 stable and there are compilation issues due to 
missed dependencies between ASoC and SoundWire subsystems.

sound/soc/intel/boards/sof_sdw.c: In function ‘sdw_startup’:
sound/soc/intel/boards/sof_sdw.c:162:9: error: implicit declaration of 
function ‘sdw_startup_stream’; did you mean ‘sdw_enable_stream’? 
[-Werror=implicit-function-declaration]
   162 |  return sdw_startup_stream(substream);
       |         ^~~~~~~~~~~~~~~~~~
       |         sdw_enable_stream
sound/soc/intel/boards/sof_sdw.c: In function ‘sdw_shutdown’:
sound/soc/intel/boards/sof_sdw.c:167:2: error: implicit declaration of 
function ‘sdw_shutdown_stream’; did you mean ‘sdw_shutdown’? 
[-Werror=implicit-function-declaration]
   167 |  sdw_shutdown_stream(substream);
       |  ^~~~~~~~~~~~~~~~~~~
       |  sdw_shutdown

I also tried with plain vanilla v5.9-rc1, and I didn't see any issues. I 
think it'd be good to have this patch (and the other one with the 
namespace) be added to v5.9 indeed.

That said, this just adds compilation/build support. We are still 
missing PM patches that were merged by Vinod earlier this week, we 
submitted the multi-link patches and the last batch will be the 
bandwidth allocation.

UCM files + PulseAudio changes were shared earlier, so at this point 
5.10 should hopefully be the first intercept where SoundWire 'just 
works'(tm) across kernel/userspace.