dt-bindings: rng: apm,x-gene-rng: convert APM RNG to dtschema

Convert the APM X-Gene 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/20211227183251.132525-1-krzysztof.kozlowski@canonical.com