ASoC: max98357a: Document MAX98357A bindings

Add documentation to the sound directory of the
device-tree bindings for the Maxim MAX98357A audio
DAC.

Signed-off-by: Kenneth Westfield <kwestfie@codeaurora.org>
Acked-by: Banajit Goswami <bgoswami@codeaurora.org>
Signed-off-by: Mark Brown <broonie@kernel.org>