dt-bindings: rng: atmel,at91-trng: add sam9x7 TRNG

Add compatbile for Microchip sam9x7 TRNG.

Signed-off-by: Varshini Rajendran <varshini.rajendran@microchip.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>