dt-bindings: phy-rockchip-inno-usb2: add assign clock property in usb2-phy node

On some platform such as RK3328, the 480m clock may need to assign
clock parent in dts in stead of clock driver. So this patch add
property of assigned-clocks and assigned-clock-parents to assign
parent for 480m clock.

Signed-off-by: Meng Dongyang <daniel.meng@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>