ASoC: twl6040: Add digital mute support

To reduce pop noise during playback stream start and stop the codec needs
to have the digital_mute callback implemented.
The codec need to be muted before the CPU dai has been stopped (McPDM).
Stopping the McPDM will generate a pop on the codec since no signal on the
PDM bus means full negative amplitude.
By managing the mute/unmute state of the outputs we can decrease the amount
of pop noise when playback starts or stops.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>