hwrng: brcm63xx - document device tree bindings

Documents device tree bindings for random number generator present on Broadcom
BCM6368 SoC.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>