ASoC: dapm - Add methods to retrieve snd_card and soc_card from dapm context.

In preparation for ASoC Dynamic PCM (AKA DSP) support.

Provide convenience methods to retrieve the soc_card or snd_card from a
DAPM context.

Signed-off-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>