dt-bindings: rng: add clock to DT binding documentation for hwrng

Add clock to DT binding documentation.
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Documentation/devicetree/bindings/rng/amlogic,meson-rng.txt

@@ -6,9 +6,16 @@ Required properties:
 - compatible : should be "amlogic,meson-rng"
 - reg : Specifies base physical address and size of the registers.
 
+Optional properties:
+
+- clocks : phandle to the following named clocks
+- clock-names: Name of core clock, must be "core"
+
 Example:
 
 rng {
 	compatible = "amlogic,meson-rng";
 	reg = <0x0 0xc8834000 0x0 0x4>;
+	clocks = <&clkc CLKID_RNG0>;
+	clock-names = "core";
 };