dt-bindings: rng: Incorporate brcm,bcm6368.txt binding

Since the same block is used on BCM2835 and BCM6368, merge the bindings
and remove the brcm,bcm6368.txt binding document.

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