ASoC: samsung: i2s: Add widgets and routes for DPCM support

This patch adds DAPM widgets required to model the internal mixer
of the I2S controller merging audio streams from the primary and
from the secondary PCM interface.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>