dt-bindings: arm: stm32: add required #clock-cells property

On STM32MP25 SoC, the syscfg peripheral provides a clock to the display
subsystem through a multiplexer. Since it only provides a single clock,
the cell value is 0.

Doing so allows the clock consumers to reach the peripheral and gate the
clock accordingly.

Reviewed-by: Rob Herring (Arm) <[e-mail]>
Reviewed-by: Yannick Fertre <[e-mail]>
Acked-by: Christophe Roullier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Raphael Gallais-Pou <[e-mail]>

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

···
   clocks:
     maxItems: 1
 
+  "#clock-cells":
+    const: 0
+
 required:
   - compatible
   - reg
 
-if:
-  properties:
-    compatible:
-      contains:
-        enum:
-          - st,stm32mp157-syscfg
-          - st,stm32f4-gcan
-then:
-  required:
-    - clocks
+allOf:
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - st,stm32mp157-syscfg
+              - st,stm32f4-gcan
+    then:
+      required:
+        - clocks
+  - if:
+      properties:
+        compatible:
+          const: st,stm32mp25-syscfg
+    then:
+      required:
+        - "#clock-cells"
 
 additionalProperties: false