ASoC: mediatek: mt8183-da7219: add speaker switch

Da7219 and max98357a share the same I2S lines.  When writing audio data
to the I2S, both codecs generate sound.

Da7219 already has a separate control "Headphone Switch".  Adds a new
control "Speakers Switch" for turning on/off max98357a.  Userspace
program can decide to turn on/off which codecs by different use cases.

Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20200212124608.2.I5fa3fdca69dbb5d3dd5031c939b9b24095065a94@changeid
Signed-off-by: Mark Brown <broonie@kernel.org>