ASoC: Add helper functions to cast from DAPM context to CODEC/platform

This is useful if we have a pointer to a DAPM context and know that it is a
CODEC or platform DAPM context and want to get a pointer to the CODEC or
platform.

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