ASoC: dapm: add a dapm sink widget

DAPM models various widgets but lacks a sink widget.
DSPs can have modules which take audio data, process it and are
capable of generating events thus acting as a sink of data.

To make the dapm graph complete for such paths we need a dapm
sink widget for these modules, so add a SND_SOC_DAPM_SINK to
declare such a widget.  This widget will be treated as
SND_SOC_DAPM_EP_SINK endpoint in the dapm graph

Signed-off-by: Vinod Koul <vinod.koul@intel.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>