phy: add a driver for the Rockchip SoC internal USB2.0 PHY

This patch to add a generic PHY driver for ROCKCHIP usb PHYs,
currently this driver can support RK3288. The RK3288 SoC have
three independent USB PHY IPs which are all configured through a
set of registers located in the GRF (general register files)
module.

Signed-off-by: Yunzhi Li <lyz@rock-chips.com>
Tested-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>