dt-bindings: rng: add reset node for stm32

Adding optional resets property for rng.
Signed-off-by: NLionel Debieve <lionel.debieve@st.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/rng/st,stm32-rng.txt

@@ -11,6 +11,9 @@ Required properties:
 - interrupts : The designated IRQ line for the RNG
 - clocks : The clock needed to enable the RNG
 
+Optional properties:
+- resets : The reset to properly start RNG
+
 Example:
 
 	rng: rng@50060800 {