dt-bindings: Document STM32 HASH bindings

This adds documentation of device tree bindings for the STM32
HASH controller.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>