dt-bindings: arm: stm32: add compatible for syscon gcan node

Since commit ad440432

 ("dt-bindings: mfd: Ensure 'syscon' has a
more specific compatible") it is required to provide at least 2 compatibles
string for syscon node.
This patch documents the new compatible for stm32f4 SoC to support
global/shared CAN registers access for bxCAN controllers.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/all/20230328073328.3949796-2-dario.binacchi@amarulasolutions.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>