dt/bindings: Add bindings for hisilicon random number generator

Document the devicetree bindings for the random number generator found
on Hisilicon Hip04 and Hip05 soc.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Acked-by: Rob Herring <rob@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>