Documentation: bindings: add DT documentation for Rockchip USB2PHY

Signed-off-by: Frank Wang <frank.wang@rock-chips.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>