configs: sandbox: Enable DM_USB_GADGET

Switch sandbox to DM_USB_GADGET, DM is the future.

Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Marek Vasut <marex@denx.de>