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>

Documentation/devicetree/bindings/arm/stm32.txt

+STMicroelectronics STM32 Platforms Device Tree Bindings
+
+Each device tree must specify which STM32 SoC it uses,
+using one of the following compatible strings:
+
+  st,stm32f429
+  st,stm32f469
+  st,stm32f746
+  st,stm32h743