ASoC: Intel: add SNDRV_PCM_INFO_DRAIN_TRIGGER flag

Add SNDRV_PCM_INFO_DRAIN_TRIGGER in snd_pcm_hardware.info to
call sst_hsw_stream_set_silence_start() when PCM draining.

Signed-off-by: Libin Yang <libin.yang@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>