ARM: DT: STi: STiH407: Add dwc3 usb3 DT node.

Now that both usb2 and usb3 phy drivers, and also the ST dwc3 glue code
are all present upstream, we can add the dwc3 DT node and have a working
usb3 controller on stih407-b2120 and stih410-b2020.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>