Documentation: dt-bindings: add dt binding info for Rockchip dwc2

This add necessary dwc2 binding documentation for Rockchip socs:
rk3066, rk3188 and rk3288

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>