phy: phy-stih407-usb: Add dt documentation for USB picophy found on stih407 SoC family

This patch adds the dt documentation for the usb picophy found on stih407 SoC family
available from STMicroelectronics.

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>