phy: phy-stih407-usb: Add usb picoPHY driver found on stih407 SoC family

This is the generic phy driver for the picoPHY ports used by the
USB2 and USB3 Host controllers when controlling usb2/1.1 devices. It
is found on STiH407 SoC family 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>