mcp3021 properties

Required properties:
- compatible: Must be one of the following:
	- "microchip,mcp3021" for mcp3021
	- "microchip,mcp3221" for mcp3221
- reg: I2C address

Optional properties:

- reference-voltage-microvolt
	Reference voltage in microvolt (uV)

Example:

mcp3021@4d {
	compatible = "microchip,mcp3021";
	reg = <0x4d>;

	reference-voltage-microvolt = <4500000>; /* 4.5 V */
};