Existing U-Boot musb driver has no support for the new gadget framework
and also seems to have other limitations. As gadget framework is ported
from Linux it seems pretty natural to port musb gadget driver as well.

This driver supports both host and peripheral modes.

This is not a replacement for current musb driver (at least now) as
there are still some consumers of the old UDC interface.

No DMA operation support included, CONFIG_MUSB_PIO_ONLY should be
defined.

Virtual root hub device is not implemented.

Known problems: with no devices connected usb_lowlevel_start() fails.

Signed-off-by: Ilya Yanok <ilya.yanok@cogentembedded.com>