dt-bindings: rng: st,rng: convert ST RNG to dtschema

Convert the ST RNG bindings to DT schema.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211227183251.132525-7-krzysztof.kozlowski@canonical.com