Documentation sysfs-bus-usb: Add rx_lanes and tx_lanes introduced in USB 3.2

rx_lanes and tx_lanes sysfs entries show the number of lanes in use by a
device.
USB 3.2 adds support for Dual-lane (symmetrical), using 2 rx lanes and
2 tx lanes for normal non Inter-Chip SSIC devices.
USB 3.1 and older are all single lane.

SSIC devices can have up to 4 lanes per direction in use,
with different number of rx and tx lanes.

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>