Call onboard_hub_create/destroy_pdevs() from hub_probe/disconnect()
to create/destroy platform devices for onboard USB hubs that may be
connected to the hub. The onboard hubs must have nodes in the
device tree.

onboard_hub_create/destroy_pdevs() are NOPs unless
CONFIG_USB_ONBOARD_HUB=y/m.

Also add a field to struct usb_hub to keep track of the onboard hub
platform devices that are owned by the hub.
Reviewed-by: NDouglas Anderson <dianders@chromium.org>
Signed-off-by: NMatthias Kaehlcke <mka@chromium.org>
Link: https://lore.kernel.org/r/20220630123445.v24.4.Ic9dd36078f9d803de82ca01a6700c58b8e4de27e@changeidSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>