ARM: dts: stih410: Enable USB2.0 and related PHY nodes at board level

A board might not expose the USB2.0 ports, so disable them by default in SoC
file, and enable them in b2120 board.

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