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>

Documentation/devicetree/bindings/sound/max98357a.txt

+Maxim MAX98357A audio DAC
+
+This node models the Maxim MAX98357A DAC.
+
+Required properties:
+- compatible   : "maxim,max98357a"
+- sdmode-gpios : GPIO specifier for the GPIO -> DAC SDMODE pin
+
+Example:
+
+max98357a {
+	compatible = "maxim,max98357a";
+	sdmode-gpios = <&qcom_pinmux 25 0>;
+};