usb: otg: mxs-phy: add missing type to usb_phy

The mxs-phy is missing the phy.type property, why the usb_get_phy helper
function won't be able to find it. This patch adds this missing property.

Signed-off-by: Michael Grzeschik <mgr@pengutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>