dt-bindings: stm32: add bindings for ML-AHB interconnect

Document the ML-AHB interconnect for stm32 SoCs.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>