hwrng: exynos - add Samsung Exynos True RNG driver

Add support for True Random Number Generator found in Samsung Exynos
5250+ SoCs.

Signed-off-by: Łukasz Stelmach <l.stelmach@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Philippe Ombredanne <pombredanne@nexb.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>