dt-bindings: rng: nuvoton,npcm-rng: Add npcm845 compatible string

Add a compatible string for Nuvoton BMC NPCM845 RNG.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>