dm: Disable dm_warn() in SPL

Since this function can use up quite a bit of space for its strings, disable
it by default in SPL. Use CONFIG_DM_WARN to re-enable it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@ti.com>

include/config_defaults.h

@@ -21,6 +21,7 @@
 #define CONFIG_PARTITIONS 1
 
 #ifndef CONFIG_SPL_BUILD
+#define CONFIG_DM_WARN
 #define CONFIG_DM_DEVICE_REMOVE
 #define CONFIG_DM_STDIO
 #endif