ASoC: ak5558: Add bindings for AK5558 ADC

Document the bindings for AK5558 ADC.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

Documentation/devicetree/bindings/sound/ak5558.txt

+AK5558 8 channel differential 32-bit delta-sigma ADC
+
+This device supports I2C mode only.
+
+Required properties:
+
+- compatible : "asahi-kasei,ak5558"
+- reg : The I2C address of the device.
+
+Optional properties:
+
+- reset-gpios: A GPIO specifier for the power down & reset pin.
+
+Example:
+
+&i2c {
+	ak5558: adc@10 {
+		compatible = "asahi-kasei,ak5558";
+		reg = <0x10>;
+		reset-gpios = <&gpio1 10 GPIO_ACTIVE_LOW>;
+	};
+};