ASoC: Intel: create boards folder and move sst boards files in

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

Signed-off-by: Jie Yang <yang.jie@intel.com>
Acked-by: Vinod Koul <vinod.koul@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>