usb: musb: jz4740: Let the platform probe the PHY

By registering a generic USB PHY from within the driver, we may shadow
the USB PHY registered by the platform, which might be different.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>