dt-bindings: usb: Document the STM32F7xx DWC2 USB OTG HS core binding

This patch adds binding documentation for DWC2 controller in HS mode found
on STMicroelectronics STM32F7xx SoC.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>