phy: rockchip-usb: should be a child device of the GRF

The usb-phy is fully enclosed in the general register files (GRF).
Therefore as seen from the device-tree it shouldn't be a separate
platform-device but instead a sub-device of the GRF - using the
simply-mfd mechanism.

As the usb-phy is part of the kernel for some releases now, we keep
the old (and now deprecated) binding for compatibility purposes.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>