dt-bindings: rng: add bindings for Xiphera XIP8001B hwrng

Document the device tree bindings of Xiphera's XIP8001B-trng IP.

Signed-off-by: Atte Tommiska <atte.tommiska@xiphera.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>