ASoC: Intel: create common folder and move common files in

Restructure the sound/soc/intel/ directory: create common folder, and move
sst common files here.

Signed-off-by: Jie Yang <yang.jie@intel.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>