ASoC: core: Add helper for DAI widgets linking

Add a helper for DAI widgets linking in preparation for
DAI-multicodec support.

No functional change.

Signed-off-by: Misael Lopez Cruz <misael.lopez@ti.com>
[fparent@baylibre.com: Adapt to 3.14+]
Signed-off-by: Fabien Parent <fparent@baylibre.com>
Signed-off-by: Benoit Cousson <bcousson@baylibre.com>
Signed-off-by: Mark Brown <broonie@linaro.org>