dt/bindings: exynos-rng: Move dt binding documentation to bindings/crypto

Samsung exynos PRNG driver is using crypto framework instead of
hw_random framework. So move the devicetree binding to crypto folder.

Signed-off-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>