dt-bindings: Document the STM32 HW RNG bindings

This adds documentation of device tree bindings for the STM32 hardware
random number generator.
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Acked-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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

+STMicroelectronics STM32 HW RNG
+===============================
+
+The STM32 hardware random number generator is a simple fixed purpose IP and
+is fully separated from other crypto functions.
+
+Required properties:
+
+- compatible : Should be "st,stm32-rng"
+- reg : Should be register base and length as documented in the datasheet
+- interrupts : The designated IRQ line for the RNG
+- clocks : The clock needed to enable the RNG
+
+Example:
+
+	rng: rng@50060800 {
+		compatible = "st,stm32-rng";
+		reg = <0x50060800 0x400>;
+		interrupts = <80>;
+		clocks = <&rcc 0 38>;
+	};