ARM: DT: STi: stihxxx-b2120: Enable USB3 port on stih407-b2120 and stih410-b2120

The USB3 controller is present on both variants of the b2120 board so
enable the controller in the generic stihxxx-b2120.dtsi file.

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