ASoC: da7218: Enable mic level detection reporting to user-space

This patch adds support to the codec driver to handle mic level
detect related IRQs, and report these to user-space using a uevent
variable.

The uevent variable string "EVENT=MIC_LEVEL_DETECT" is sent to
user-space, if the mic level detect feature is enabled, and the
audio captured at the chosen mic(s) is above a certain threshold.
User-space can then handle the event accordingly (e.g. process
audio capture stream).

This method was chosen over ALSA control notification for a couple
of reasons:

 1) There's no requirement here for a control to read state from.
    The event is the only thing that's required and of interest.
 2) tinyalsa support for control notifications does not exist so on
    platforms using this over alsa-lib there is a need to add code
    to support this event handling.

Another possible option would be to use the standard Jack reporting
framework but this really does not fit for this kind of event.

Finally, use of the input device framework is not being encouraged,
due to difficulties in enabling apps to access input devices, so
this has also been avoided.

Signed-off-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Signed-off-by: Mark Brown <broonie@kernel.org>