usb: add device connection/disconnection detection

Provide a function to detect USB device insertion/removal in order to
avoid having to do USB enumeration in a tight loop when trying to detect
peripheral hotplugging.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>

Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
Tested-by: Stefan Reinauer <reinauer@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>