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>

arch/arm/boot/dts/stihxxx-b2120.dtsi

@@ -74,5 +74,10 @@ phy_port1: port@9b2a000 {
 				st,osc-force-ext;
 			};
 		};
+
+		st_dwc3: dwc3@8f94000 {
+			status = "okay";
+		};
+
 	};
 };