devicetree: bindings: Document supported STM32 SoC family

This adds a list of supported STM32 SoC bindings.

Signed-off-by: Gwenael Treuveur <gwenael.treuveur@st.com>
Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>