usb: dwc3: rockchip: add devicetree bindings documentation

This patch adds the devicetree documentation required for Rockchip
USB3.0 core wrapper consisting of USB3.0 IP from Synopsys.

It supports DRD mode, and could operate in device mode (SS, HS, FS)
and host mode (SS, HS, FS, LS).

Signed-off-by: William Wu <william.wu@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>