dt-bindings: crypto: qcom,prng: document SA8775P and SC7280

Document SA8775P and SC7280 compatible for the True Random Number
Generator.

Signed-off-by: Om Prakash Singh <quic_omprsing@quicinc.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>