usb: xhci: Program 'route string' in the input slot context

usb: xhci: Program 'route string' in the input slot context

xHCI spec says: the values of the 'route string' field shall be
initialized by the first 'Address Device' command issued to a
device slot, and shall not be modified by any other command.

So far U-Boot does not program this field, and it does not prevent
SS device directly attached to root port, or HS device behind an HS
hub, from working, due to the fact that 'route string' is used by
the xHC to target SS packets. But in order to enumerate devices
behind an SS hub, this field must be programmed.

With this commit and along with previous commits, now SS & HS devices
attached to a USB 3.0 hub can be enumerated by U-Boot.

As usual, this new feature is only available when DM is on.

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