dm: usb: Add support for companion controllers

USB companion controllers must be scanned after the main controller has
been scanned, so that any devices which the main controller which to hand
over to the companion have actually been handed over before we scan the
companion.

As there are no guarantees that this will magically happen in the right
order, split the scanning of the buses in 2 phases, first main controllers,
and then companion controllers.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Simon Glass <sjg@chromium.org>