ASoC: Add external amplifier controls for Visstrim_M10.

Visstrim_M10 has an external class D amplifier.
This patch provides support for controlling the 4
possible gain levels and per channel muting.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>