ASoC: ak4458: Add bindings for AK4458 DAC

Document the bindings for AK4458 DAC
Signed-off-by: Cosmin-Gabriel Samoila <cosmin.samoila@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/ak4458.txt

+AK4458 audio DAC
+
+This device supports I2C mode.
+
+Required properties:
+
+- compatible : "asahi-kasei,ak4458"
+- reg : The I2C address of the device for I2C
+
+Optional properties:
+- reset-gpios: A GPIO specifier for the power down & reset pin
+- mute-gpios: A GPIO specifier for the soft mute pin
+
+Example:
+
+&i2c {
+	ak4458: dac@10 {
+		compatible = "asahi-kasei,ak4458";
+		reg = <0x10>;
+		reset-gpios = <&gpio1 10 GPIO_ACTIVE_LOW>
+		mute-gpios = <&gpio1 11 GPIO_ACTIVE_HIGH>
+	};
+};