usb: dwc3: dwc3-st: Add st-dwc3 devicetree bindings documentation

This patch documents the device tree documentation required for
the ST usb3 controller glue layer found in STiH407 devices.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>