usb: gadget: composite: add USB_DT_OTG request handling

Copy usb_otg_descriptor from config's descriptor if host requests
USB_DT_OTG.

Signed-off-by: Macpaul Lin <macpaul@gmail.com>
Signed-off-by: Li Jun <jun.li@freescale.com>
Reviewed-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>