usb: gadget: libcomposite: move composite.c into libcomposite

This moves composite.c into libcomposite and updates all gadgets.
Finally!

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>