ASoC: Intel: Skylake: Add channel constraints for refcap

Add constraint for ref DMIC to match with the
topology firmware config.

Signed-off-by: Yong Zhi <yong.zhi@intel.com>
Acked-by: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>