dt-bindings: rng: bcm2835: add clock constraints

brcm,bcm6368-rng controllers require enabling the IPSEC clock in order to get
a functional RNG.

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