dt-bindings: rng: atmel,at91-trng: convert Atmel TRNG to dtschema

Convert the Atmel TRNG bindings to DT schema.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211227183251.132525-2-krzysztof.kozlowski@canonical.com