usb: gadget: uvc: configfs: Allocate groups dynamically

The UVC configfs implementation creates all groups as global static
variables. This prevents creation of multiple UVC function instances,
as they would all require their own configfs group instances.

Fix this by allocating all groups dynamically. To avoid duplicating code
around, extend the config_item_type structure with group name and
children, and implement helper functions to create children
automatically for most groups.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>