dt-bindings: Document STM32 CRYP bindings

Document device tree bindings for the STM32 CRYP.

Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>