Voltage divider
===============

When an io-channel measures the midpoint of a voltage divider, the
interesting voltage is often the voltage over the full resistance
of the divider. This binding describes the voltage divider in such
a curcuit.

    Vin ----.
            |
         .-----.
         |  R  |
         '-----'
            |
            +---- Vout
            |
         .-----.
         | Rout|
         '-----'
            |
           GND

Required properties:
- compatible : "voltage-divider"
- io-channels : Channel node of a voltage io-channel measuring Vout.
- output-ohms : Resistance Rout over which the output voltage is measured.
	        See full-ohms.
- full-ohms : Resistance R + Rout for the full divider. The io-channel
	      is scaled by the Rout / (R + Rout) quotient.

Example:
The system voltage is circa 12V, but divided down with a 22/222
voltage divider (R = 200 Ohms, Rout = 22 Ohms) and fed to an ADC.

sysv {
	compatible = "voltage-divider";
	io-channels = <&maxadc 1>;

	/* Scale the system voltage by 22/222 to fit the ADC range. */
	output-ohms = <22>;
	full-ohms = <222>; /* 200 + 22 */
};

&spi {
	maxadc: adc@0 {
		compatible = "maxim,max1027";
		reg = <0>;
		#io-channel-cells = <1>;
		interrupt-parent = <&gpio5>;
		interrupts = <15 IRQ_TYPE_EDGE_RISING>;
		spi-max-frequency = <1000000>;
	};
};