ASoC: max9867: add mono playback switch

Add Mono Playback switch to codec controls which enables codec's
capability of mixing left and right channel at the DAC output.

Signed-off-by: Pavel Dobias <dobias@2n.cz>
Link: https://lore.kernel.org/r/20200520071904.15801-5-dobias@2n.cz
Signed-off-by: Mark Brown <broonie@kernel.org>