ASoC: SOF: topology: add min/max step for volume_table

add two units min_volume_step and max_volume_step to the snd_sof_control
struct, for the min and max step of the volume_table.

Signed-off-by: Zhu Yingjiang <yingjiang.zhu@linux.intel.com>
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>