ASoC: Intel: Add ASoC Intel SST Atom sysfs description

For ASoC Intel SST Atom based devices a sysfs entry is created
in order to track FW version. The FW version is useful in order to
check the different Version of LPE DSP FW across Intel SoCs.
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>

Documentation/ABI/testing/sysfs-platform-sst-atom

+What:		/sys/devices/platform/8086%x:00/firmware_version
+Date:		November 2016
+KernelVersion:	4.10
+Contact:	"Sebastien Guiriec" <sebastien.guiriec@intel.com>
+Description:
+		LPE Firmware version for SST driver on all atom
+		plaforms (BYT/CHT/Merrifield/BSW).
+		If the FW has never been loaded it will display:
+			"FW not yet loaded"
+		If FW has been loaded it will display:
+			"v01.aa.bb.cc"
+		aa: Major version is reflecting SoC version:
+			0d: BYT FW
+			0b: BSW FW
+			07: Merrifield FW
+		bb: Minor version
+		cc: Build version