ASoC: dapm: Introduce DAPM_DOUBLE dual channel control type

A DAPM_DOUBLE control type can be used for dual channel mixer input
selectors / mute controls in one register, possibly toggling both
channels together.

The control is meant to be shared by 2 widgets, 1 for each channel,
such that the mixer control exposed to userspace remains a combined
stereo control.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Mark Brown <broonie@kernel.org>