usb: gadget: add streams support to the gadget framework

This patch defines necessary fields to support
streaming for USB3.0.

It implements a new function, called
usb_ep_autoconfig_ss(), to be used instead of the
existing usb_ep_autoconfig() when working in
SuperSpeed mode and there is a need to search for
an endpoint according to the number of required
streams.

[ balbi@ti.com : slight changes to commit log ]

Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Tatyana Brokhman <tlinder@codeaurora.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>