dt-bindings: crypto: Add DT bindings documentation for sun8i-ss Security System

This patch adds documentation for Device-Tree bindings of the
Security System cryptographic offloader driver.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>