ASoC: Intel: atom: Add sysfs entry in order to store FW version

This patch is adding a sysfs entry in order to be able to get
access to SST FW version.

Signed-off-by: Sebastien Guiriec <sebastien.guiriec@intel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>