dt-bindings: usb: renesas_usb3: Document usb role switch support

Update the DT bindings documentation to support usb role switch
for USB Type-C connector using USB role switch class framework.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1567584941-13690-4-git-send-email-biju.das@bp.renesas.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>