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>
diff --git a/configs/sandbox_spl_defconfig b/configs/sandbox_spl_defconfig
index 69b4133..4a67af2 100644
--- a/configs/sandbox_spl_defconfig
+++ b/configs/sandbox_spl_defconfig
@@ -223,6 +223,7 @@
 CONFIG_TIMER_EARLY=y
 CONFIG_SANDBOX_TIMER=y
 CONFIG_USB=y
+CONFIG_DM_USB_GADGET=y
 CONFIG_USB_EMUL=y
 CONFIG_USB_KEYBOARD=y
 CONFIG_VIDEO=y