ASoC: SOF: use the sof_ipc_comp reserved bytes for extended data

Use the 32bit reserved member of the struct sof_ipc_comp as the extended
data length, this will help to minimize the ABI change for adding new
extended data to the struct sof_ipc_comp, usually only minor ABI version
bump needed for every update with this new solution.
Signed-off-by: Keyon Jie <yang.jie@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Link: https://lore.kernel.org/r/20200904132744.1699575-6-kai.vehmanen@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>

include/sound/sof/topology.h

@@ -57,8 +57,8 @@ struct sof_ipc_comp {
 	uint32_t pipeline_id;
 	uint32_t core;
 
-	/* reserved for future use */
-	uint32_t reserved[1];
+	/* extended data length, 0 if no extended data */
+	uint32_t ext_data_length;
 } __packed;
 
 /*