dt-bindings: net: can: fix dtbs warning

Mute the warning from "make dtbs_check":

Documentation/devicetree/bindings/net/can/bosch,m_can.example.dt.yaml:
can@20e8000: bosch,mram-cfg: [[0, 0, 0, 32, 0, 0, 0, 1]] is too short
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220128133142.2135718-1-dinguyen@kernel.org

Documentation/devicetree/bindings/net/can/bosch,m_can.yaml

@@ -101,6 +101,7 @@ properties:
       - description: Tx Buffers 0-32 elements / 0-576 words
         minimum: 0
         maximum: 32
+    minItems: 1
 
   power-domains:
     description: