usb:gadget:composite USB composite gadget support

USB Composite gadget implementation for u-boot. It builds on top
of USB UDC drivers.

This commit is based on following files from Linux Kernel v2.6.36:

./include/linux/usb/composite.h
./drivers/usb/gadget/composite.c

SHA1: d187abb9a83e6c6b6e9f2ca17962bdeafb4bc903

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