usb: lowlevel interface change to support multiple controllers

Carry an index in the lowlevel usb functions to make specify the
respective usb controller.

Also pass through an controller struct from lowlevel_init to the
creation of the root usb device of this controller.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Reviewed-by: Marek Vasut <marex@denx.de>