dt-bindings: crypto: add documentation for aspeed hace

Add device tree binding documentation for the Aspeed Hash
and Crypto Engines (HACE) Controller.

Signed-off-by: Neal Liu <neal_liu@aspeedtech.com>
Signed-off-by: Johnny Huang <johnny_huang@aspeedtech.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>