usb:gadget:composite: Support for composite at gadget.h

Add device data pointer to the USB gadget's device struct.
Wrapper for extracting usb_gadget from Linux's usb device

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Marek Vasut <marex@denx.de>