[media] dt-bindings: Document STM32 DCMI bindings

This adds documentation of device tree bindings for the STM32 DCMI
(Digital Camera Memory Interface).

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>