dt-bindings: net: can: add STM32 bxcan DT bindings

Add documentation of device tree bindings for the STM32 basic extended
CAN (bxcan) controller.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/all/20230328073328.3949796-3-dario.binacchi@amarulasolutions.com
[mkl: drop unneeded quotes]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>