* AT91's Analog to Digital Converter (ADC)

Required properties:
  - compatible: Should be "atmel,at91sam9260-adc"
  - reg: Should contain ADC registers location and length
  - interrupts: Should contain the IRQ line for the ADC
  - atmel,adc-channel-base: Offset of the first channel data register
  - atmel,adc-channels-used: Bitmask of the channels muxed and enable for this
    device
  - atmel,adc-drdy-mask: Mask of the DRDY interruption in the ADC
  - atmel,adc-num-channels: Number of channels available in the ADC
  - atmel,adc-startup-time: Startup Time of the ADC in microseconds as
    defined in the datasheet
  - atmel,adc-status-register: Offset of the Interrupt Status Register
  - atmel,adc-trigger-register: Offset of the Trigger Register
  - atmel,adc-vref: Reference voltage in millivolts for the conversions

Optional properties:
  - atmel,adc-use-external: Boolean to enable of external triggers
 
Optional trigger Nodes:
  - Required properties:
    * trigger-name: Name of the trigger exposed to the user
    * trigger-value: Value to put in the Trigger register
      to activate this trigger
  - Optional properties:
    * trigger-external: Is the trigger an external trigger?

Examples:
adc0: adc@fffb0000 {
	compatible = "atmel,at91sam9260-adc";
	reg = <0xfffb0000 0x100>;
	interrupts = <20 4>;
	atmel,adc-channel-base = <0x30>;
	atmel,adc-channels-used = <0xff>;
	atmel,adc-drdy-mask = <0x10000>;
	atmel,adc-num-channels = <8>;
	atmel,adc-startup-time = <40>;
	atmel,adc-status-register = <0x1c>;
	atmel,adc-trigger-register = <0x08>;
	atmel,adc-use-external;
	atmel,adc-vref = <3300>;

	trigger@0 {
		trigger-name = "external-rising";
		trigger-value = <0x1>;
		trigger-external;
	};
	trigger@1 {
		trigger-name = "external-falling";
		trigger-value = <0x2>;
		trigger-external;
	};

	trigger@2 {
		trigger-name = "external-any";
		trigger-value = <0x3>;
		trigger-external;
	};

	trigger@3 {
		trigger-name = "continuous";
		trigger-value = <0x6>;
	};
};