dt-bindings: interrupt-controller: stm32: Document hwlock properties

Add hwlocks as optional property

Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Benjamin Gaignard <[e-mail]>
Signed-off-by: Marc Zyngier <[e-mail]>

Documentation/devicetree/bindings/interrupt-controller/st,stm32-exti.txt

···
   (only needed for exti controller with multiple exti under
   same parent interrupt: st,stm32-exti and st,stm32h7-exti)
 
+Optional properties:
+
+- hwlocks: reference to a phandle of a hardware spinlock provider node.
+
 Example:
 
 exti: interrupt-controller@40013c00 {