phy: phy-stih41x-usb: Add dt documentation for USB phy on STiH415/6

This patch adds dt documentation bindings for the usb phy found
on STiH415/5 SoC's from STMicroelectronics, which support USB 1.1 and 2.0.

Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>