dt-bindings: rng: exynos4-rng: Convert Exynos PRNG bindings to json-schema

Convert Samsung Exynos Pseudo Random Number Generator bindings to DT
schema format using json-schema.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>