ASoC: arizona: Add voice trigger output widget

In some situations the voice control firmware will by used
to only provide a trigger notification event. In this case a
compressed stream will not be opened by user-space, as such we
need to provide a virtual output to power on the DSP in this
use-case. This patch adds a virtual output 'DSP Voice Trigger'
that can be used for this, and a switch that lets it be connected
to the core when required.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>