usb: gadget: add USB3 support to the printer driver

Add SS descriptors to support the capabilities provided by USB3 controller
drivers; unit tests run using a PLX 3380 [max transfer speed measured of 1Gbps]

This driver shall fallback to lower operating modes when the higher ones are
not available.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>