usb: xhci: Fix root hub descriptor

When a system has multiple XHCI controllers, some of the
properties described in the descriptor of the root hub (such as
the number of ports) might differ between controllers.  Fix this
by switching from a single global hub descriptor to a hub
descriptor per controller.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Reviewed-by: Marek Vasut <marex@denx.de>