phy: rockchip-inno-usb2: add support for rockchip,usbgrf property

The registers of usb-phy are distributed in grf and usbgrf on some
Rockchip SoCs (e.g RV1108), this patch add a new rockchip,usbgrf
property to support this companion grf design.

Signed-off-by: Frank Wang <frank.wang@rock-chips.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>