usb: phy: phy-mxs-usb: add imx7ulp support

At imx7ulp, the USB related analog register is located in PHY register
region too, so we need to control PLL at PHY driver directly.

Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>