DM is the modern default approach for the drivers in U-Boot.
It also allows to configure code via Device Tree.

Move Intel Edison to use DM_USB_GADGET and drop hard coded values.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>