usb_dev
the usb root hub device to be registered.
parent_dev
the parent device of this root hub.
The USB host controller calls this function to register the root hub
properly with the USB subsystem. It sets up the device properly in
the device tree and stores the root_hub pointer in the bus structure,
then calls usb_new_device
to register the usb device. It also
assigns the root hub's USB address (always 1).