usb: gadget: serial: allocate and init otg descriptor by otg capabilities

Allocate and initialize usb otg descriptor according to gadget otg
capabilities, add it for each usb configurations. If otg capability
is not defined, keep its original otg descriptor unchanged.

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