dt-bindings: extcon: Add support for cros-ec device

This patch add documentation for binding of USB Type C cable detection
mechanism is using EXTCON subsystem. The device can detect the presence
of display out but it may also detect other external accessories when
external accessories is attached or detached.
Signed-off-by: Benson Leung <bleung@chromium.org>
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>

Documentation/devicetree/bindings/extcon/extcon-usbc-cros-ec.txt

+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>;
+		};
+	}