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

Since commit ad440432d1f9 ("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 <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]
Signed-off-by: Marc Kleine-Budde <[e-mail]>

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

···
               - st,stm32-syscfg
               - st,stm32-power-config
               - st,stm32-tamp
+              - st,stm32f4-gcan
           - const: syscon
       - items:
           - const: st,stm32-tamp
···
       contains:
         enum:
           - st,stm32mp157-syscfg
+          - st,stm32f4-gcan
 then:
   required:
     - clocks