hwrng: xiphera-trng: add support for XIP8001B hwrng

Xiphera XIP8001B is an FPGA-based True Random Number Generator
Intellectual Property (IP) Core which can be instantiated in
multiple FPGA families. This driver adds Linux support for it through
the hwrng interface.

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