ASoC: Add DT bindings documentation for max9759 amplifier

This patch adds DT bindings documentation for Maxim MAX9759
speaker amplifier.
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/maxim,max9759.txt

+Maxim MAX9759 Speaker Amplifier
+===============================
+
+Required properties:
+- compatible : "maxim,max9759"
+- shutdown-gpios : the gpio connected to the shutdown pin
+- mute-gpios : the gpio connected to the mute pin
+- gain-gpios : the 2 gpios connected to the g1 and g2 pins
+
+Example:
+
+max9759: analog-amplifier {
+	compatible = "maxim,max9759";
+	shutdown-gpios = <&gpio3 20 GPIO_ACTIVE_LOW>;
+	mute-gpios = <&gpio3 19 GPIO_ACTIVE_LOW>;
+	gain-gpios = <&gpio3 23 GPIO_ACTIVE_LOW>,
+		     <&gpio3 25 GPIO_ACTIVE_LOW>;
+};