dt-bindings: Document the STM32 USB OTG DWC2 core binding

This patch adds the documentation for STM32F4x9 USB OTG FS/HS compatible
strings.
Signed-off-by: Bruno Herrera <bruherrera@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>

Documentation/devicetree/bindings/usb/dwc2.txt

@@ -14,6 +14,10 @@ Required properties:
   - "amlogic,meson-gxbb-usb": The DWC2 USB controller instance in Amlogic S905 SoCs;
   - "amcc,dwc-otg": The DWC2 USB controller instance in AMCC Canyonlands 460EX SoCs;
   - snps,dwc2: A generic DWC2 USB controller with default parameters.
+  - "st,stm32f4x9-fsotg": The DWC2 USB FS/HS controller instance in STM32F4x9 SoCs
+  configured in FS mode;
+  - "st,stm32f4x9-hsotg": The DWC2 USB HS controller instance in STM32F4x9 SoCs
+  configured in HS mode;
 - reg : Should contain 1 register range (address and length)
 - interrupts : Should contain 1 interrupt
 - clocks: clock provider specifier