usb: gadget: f_fs: Add support for SuperSpeed Mode

Allow userspace to pass SuperSpeed descriptors and
handle them in the driver accordingly.
This change doesn't modify existing desc_header and thereby
keeps the ABI changes backward compatible i.e. existing
userspace drivers compiled with old header (functionfs.h)
would continue to work with the updated kernel.

Change-Id: Ic27035fdef2a83828024348d75be1518e9f8c5c6
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>