Caution
If you're looking for Microchip MCP23018 linux kernel module - use upstream pinctrl-mcp23s08 module instead.
This module is strictly for learning purposes!
Device Tree section for MCP23018 module, make sure to put it into appropriate i2c section:
mcp23018: gpio@20 {
compatible = "microchip,mcp23018";
reg = <0x20;
gpio-controller;
interrupt-controller;
};