* Palmas general purpose ADC IP block devicetree bindings

Required properties:
- compatible : Must be "ti,palmas-gpadc".

Optional sub-nodes:
ti,channel0-current-microamp: Channel 0 current in uA.
	Valid values 0uA, 5uA, 15uA, 20uA.
ti,channel3-current-microamp: Channel 3 current in uA.
	Valid value 0uA, 10uA, 400uA, 800uA.
ti,enable-channel3-dual-current: Enable dual current on channel 3.
ti,enable-extended-delay: Enable extended delay.

Optional sub-node:
The Palmas ADC node has optional subnode to define the iio mapping.
It is the name with "iio_map". This node has again subnode to define
the property of the channel. The sub subnode has following properties:
- ti,adc-channel-number: ADC channel numbber.
- ti,adc-consumer-device: Consumer device name.
- ti,adc-consumer-channel: ADC consumer channel name.

Example:

pmic {
	compatible = "ti,twl6035-pmic", "ti,palmas-pmic";
	...
	gpadc {
		compatible = "ti,palmas-gpadc";
		interrupts = <18 0
			      16 0
			      17 0>;
		ti,channel0-current-microamp = <5>;
		ti,channel3-current-microamp = <10>;
		iio_map {
			ch1 {
				ti,adc-channel-number = <1>;
				ti,adc-consumer-device = "generic-adc-thermal.0";
				ti,adc-consumer-channel ="battery-temp-channel";
			};

			ch6 {
				ti,adc-channel-number = <6>;
				ti,adc-consumer-device = "palmas-battery";
				ti,adc-consumer-channel ="vbat_channel";
			};
		};
	};
	...
};