usb: phy: mv_u3d: Add usb phy driver for mv_u3d

The driver supports phy_init and phy_shutdown functions to
enable and disable phy for Marvell USB 3.0 controller.

Signed-off-by: Yu Xu <yuxu@marvell.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>