sandbox: config: Enable driver model

Use driver model in sandbox to permit running of driver model unit test.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/configs/sandbox.h

@@ -20,6 +20,7 @@
 
 #define CONFIG_BOOTSTAGE
 #define CONFIG_BOOTSTAGE_REPORT
+#define CONFIG_DM
 
 /* Number of bits in a C 'long' on this architecture */
 #define CONFIG_SANDBOX_BITS_PER_LONG	64