sandbox: defconfig: enable CONFIG_SM option

We use this option to test UCLASS_SM.

Signed-off-by: Alexey Romanov <avromanov@salutedevices.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Link: https://lore.kernel.org/r/20230921081346.22157-6-avromanov@salutedevices.com
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 01830c7..47417cb 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -280,6 +280,7 @@
 CONFIG_SCSI=y
 CONFIG_DM_SCSI=y
 CONFIG_SANDBOX_SERIAL=y
+CONFIG_SM=y
 CONFIG_SMEM=y
 CONFIG_SANDBOX_SMEM=y
 CONFIG_SOUND=y