dt-bindings: can: mcp251x: document GPIO support

The next patch adds gpio controller support to the mcp251x driver. This
patch updates the binding accordingly.

Cc: Timo Schlüßler <schluessler@krause.de>
Link: https://lore.kernel.org/r/20200915223527.1417033-26-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

Documentation/devicetree/bindings/net/can/microchip,mcp251x.txt

@@ -12,6 +12,9 @@ Required properties:
 Optional properties:
  - vdd-supply: Regulator that powers the CAN controller.
  - xceiver-supply: Regulator that powers the CAN transceiver.
+ - gpio-controller: Indicates this device is a GPIO controller.
+ - #gpio-cells: Should be two. The first cell is the pin number and
+                the second cell is used to specify the gpio polarity.
 
 Example:
 	can0: can@1 {
@@ -22,4 +25,6 @@ Example:
 		interrupts = <13 IRQ_TYPE_LEVEL_LOW>;
 		vdd-supply = <&reg5v0>;
 		xceiver-supply = <&reg5v0>;
+		gpio-controller;
+		#gpio-cells = <2>;
 	};