usb: gadget: zero: add superspeed support

This patch adds SuperSpeed descriptors to the
g_zero gadget.

The SuperSpeed descriptors were added both for
f_soursesink and f_loopback function drivers.

Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: Amit Blay <ablay@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>