ASoC: dapm: Add snd_soc_dapm_add_path() helper function

snd_soc_dapm_add_path() is similar to snd_soc_dapm_add_route() except that it
expects the pointer to the source and sink widgets instead of their names. This
allows us to simplify the case where we already have a pointer to widgets. (E.g.
as we have in snd_soc_dapm_link_dai_widgets()). snd_soc_dapm_add_route() will be
updated to just look up the widget and then use snd_soc_dapm_add_path() to
handle everything else.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@linaro.org>