hwrng: st - Add support for ST's HW Random Number Generator

Signed-off-by: Pankaj Dev <pankaj.dev@st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Kieran Bingham <kieranbingham@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>