dt-bindings: crypto: Add documentation for sl3516-ce

This patch adds documentation for Device-Tree bindings for the
SL3516-ce cryptographic offloader driver.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>