dt-bindings: usb: dwc2: add optional usb-role-switch property

This patch documents the usb-role-switch property in dwc2 bindings, now
that usb-role-switch support is available in dwc2 driver.

Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>