ChromeOS EC USB Type-C cable and accessories detection

On ChromeOS systems with USB Type C ports, the ChromeOS Embedded Controller is
able to detect the state of external accessories such as display adapters
or USB devices when said accessories are attached or detached.

The node for this device must be under a cros-ec node like google,cros-ec-spi
or google,cros-ec-i2c.

Required properties:
- compatible:		Should be "google,extcon-usbc-cros-ec".
- google,usb-port-id:	Specifies the USB port ID to use.

Example:
	cros-ec@0 {
		compatible = "google,cros-ec-i2c";

		...

		extcon {
			compatible = "google,extcon-usbc-cros-ec";
			google,usb-port-id = <0>;
		};
	}