usb: gadget: uvc: Allow definition of XUs in configfs

usb: gadget: uvc: Allow definition of XUs in configfs

The UVC gadget at present has no support for extension units. Add the
infrastructure to uvc_configfs.c that allows users to create XUs via
configfs. These will be stored in a new child of uvcg_control_grp_type
with the name "extensions".

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Daniel Scally <dan.scally@ideasonboard.com>
Link: https://lore.kernel.org/r/20230206161802.892954-4-dan.scally@ideasonboard.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>