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: NDario Binacchi <dario.binacchi@amarulasolutions.com>
Acked-by: NRob Herring <robh@kernel.org>
Link: https://lore.kernel.org/all/20230328073328.3949796-2-dario.binacchi@amarulasolutions.comSigned-off-by: NMarc Kleine-Budde <mkl@pengutronix.de>

Documentation/devicetree/bindings/arm/stm32/st,stm32-syscon.yaml

@@ -20,6 +20,7 @@ properties:
               - st,stm32-syscfg
               - st,stm32-power-config
               - st,stm32-tamp
+              - st,stm32f4-gcan
           - const: syscon
       - items:
           - const: st,stm32-tamp
@@ -42,6 +43,7 @@ if:
       contains:
         enum:
           - st,stm32mp157-syscfg
+          - st,stm32f4-gcan
 then:
   required:
     - clocks