AK5558 audio ADC

This device supports I2C mode only.

Required properties:

- compatible : "asahi-kasei,ak5558"
- reg : The I2C address of the device for I2C.
- asahi-kasei,pdn-gpios: A GPIO specifier for the GPIO controlling
	the power down & reset pin.

Example:

&i2c {
	ak5558: ak5558@10 {
		compatible = "asahi-kasei,ak5558";
		reg = <0x10>;
		asahi-kasei,pdn-gpios = <&gpio1 10 GPIO_ACTIVE_HIGH>;
	};
};