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>