dt-bindings: omap-rng: Document SafeXcel IP-76 device variant

This commits add missing fields in the documentation that are used
by the new device variant. It also includes DT example to show how
the variant should be used.

Signed-off-by: Romain Perier <romain.perier@free-electrons.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>