usb: gadget: add SS descriptors to Ethernet gadget

Add SuperSpeed descriptors to the Network USB
function drivers.

This has been lightly tested using a Linux host.
I was able to ssh from device to host and host to
device, no obvious problems seen.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>