ARM: STi: DT: STiH407: Add usb2 picophy dt nodes

This patch adds the dt nodes for the usb2 picophy found on the stih407
device family. It is used on stih407 by the dwc3 usb3 controller when
controlling usb2/1.1 devices.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>