dt-bindings: add DT bindings for usb2-phy grf

Adds the device tree bindings description for usb2-phy grf
of RK3328 platform.

Signed-off-by: Meng Dongyang <daniel.meng@rock-chips.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>