usb: xhci: Enable TT to support LS/FS devices behind a HS hub

usb: xhci: Enable TT to support LS/FS devices behind a HS hub

So far LS/FS devices directly attached to xHC root port can be
successfully enumerated by xHCI driver, but if they are connected
behind a hub, the enumeration process fails to address the device.

It turns out xHCI driver still misses a part that in the device's
input slot context, all Transaction Translator (TT) related fields
are not programmed. The xHCI spec defines how to enable TT.

Now LS/FS devices like USB keyboard/mouse can be enumerated behind
a high speed hub.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>