usb: otg: add notify_connect/notify_disconnect callback

This let usb phy driver has a chance to change hw settings when connect
status change.

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Tested-by: Subodh Nijsure <snijsure@grid-net.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>