dt-bindings: usb: Add DT bindings for STUSB160x Type-C controller

Add binding documentation for the STMicroelectronics STUSB160x Type-C port
controller.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>