dt-bindings: phy-rockchip-inno-usb2: add rockchip,usbgrf property

Add rockchip,usbgrf property to support the registers of usb-phy
that are distributed in grf and usbgrf on some special Rockchip
SoCs (e.g RV1108).

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