usb: gadget: add usb3.0 descriptors to serial gadgets

This patch adds SS descriptors to the ACM & generic serial gadget. The
ACM part was tested with minicom + dummy + send / receive files over
ttyACM <=> ttyGS0.
The generic serial part (f_serial) was not tested (haven't found a
driver on the host side).
The nokia & multi gadget use HS at most.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>