hwrng: exynos - Add support for Exynos random number generator

This patch supports Exynos SOC's PRNG driver. Exynos's PRNG has 5 seeds and
5 random number outputs. Module is excuted under runtime power management control,
so it activates only while it's in use. Otherwise it will be suspended generally.
It was tested on PQ board by rngtest program.

Signed-off-by: Jonghwa Lee <jonghwa3.lee@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>