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>