dt-bindings: rng: document Silex Insight BA431 hwrng

This patch documents the device tree bindings of the BA431 hardware
random number generator.

This IP is for instance present in the Viper OEM boards sold by Silex
Insight.

Signed-off-by: Olivier Sobrie <olivier.sobrie@silexinsight.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>