dt-bindings: crypto: qcom,prng: document SM8550

Document SM8550 compatible for the True Random Number Generator.

Reviewed-by: Om Prakash Singh <quic_omprsing@quicinc.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>