hwrng: exynos - Add Device Tree support

Add Device Tree support for the driver. The Pseudo Random Number
Generator module is the same in almost all of Exynos SoCs, since
Exynos4210 (however the tests were done only on Trats2 board with
Exynos4412). There are some differences on newer Exynos Octa
(Exynos542x) SoCs.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>