hwrng: hisi - Add support for Hisilicon SoC RNG

This adds the Hisilicon Random Number Generator(RNG) support,
which is found in Hip04 and Hip05 soc.

Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>