ASoC: topology: ABI - Add sig_bits to stream caps

Kernel struct snd_soc_pcm_stream, SoC PCM stream information, needs this
field. Although current topology users don't configure this, we define it
for future extension.

Signed-off-by: Mengdong Lin <mengdong.lin@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>