ASoC: Add SND_SOC_DAPM_PIN_SWITCH controls for exposing DAPM pins

On some systems it is desirable for control for DAPM pins to be provided
to user space. This is the case with things like GSM modems which are
controlled primarily from user space, for example. Provide a helper which
exposes the state of a DAPM pin to user space for use in cases like this.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>