usb: host: xhci-plat: Create platform device for onboard hubs in probe()

Call onboard_hub_create/destroy_pdevs() from  _probe()/_remove()
to create/destroy platform devices for onboard USB hubs that may
be connected to the root hub of the controller. These functions
are a NOP unless CONFIG_USB_ONBOARD_HUB=y/m.

Also add a field to struct xhci_hcd to keep track of the onboard hub
platform devices that are owned by the xHCI.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Link: https://lore.kernel.org/r/20210609150159.v12.4.I7a3a7d9d2126c34079b1cab87aa0b2ec3030f9b7@changeid
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>