usb: phy: mxs-phy: register phy with framework

We now have usb_add_phy_dev(), so use it to register with the framework
to be able to find the phy from the USB driver.

Tested-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Reviewed-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>