dt-bindings: crypto: fsl-imx-sahara: Document the clocks

Describe the clocks (ipg and ahb) needed by Sahara block to operate.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>