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.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>