dt-bindings: rcc: stm32: select the "secure" path for stm32mp13

Like for stm32mp15, when stm32 RCC node is used to interact with a secure
context (using clock SCMI protocol), a different path has to be used for
yaml verification.

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

Documentation/devicetree/bindings/clock/st,stm32mp1-rcc.yaml

···
       contains:
         enum:
           - st,stm32mp1-rcc-secure
+          - st,stm32mp13-rcc
 then:
   properties:
     clocks: