dt-bindings: can: flexcan: add stop mode property to device tree

The FlexCAN controller can parse the stop mode property to enable CAN
self wakeup feature.

Signed-off-by: Aisheng Dong <aisheng.dong@nxp.com>
Signed-off-by: Joakim Zhang <qiangqing.zhang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>