dt-bindings: stm32: document stm32mp135f-dk board

Add new entry for stm32mp135f-dk board.

Signed-off-by: Alexandre Torgue <[e-mail]>
Acked-by: Arnd Bergmann <[e-mail]>
Acked-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/arm/stm32/stm32.yaml

···
           - const: st,stm32h750
       - items:
           - enum:
+              - st,stm32mp135f-dk
+          - const: st,stm32mp135
+      - items:
+          - enum:
               - shiratech,stm32mp157a-iot-box # IoT Box
               - shiratech,stm32mp157a-stinger96 # Stinger96
               - st,stm32mp157c-ed1