dm: Allow stdio registration to be dropped

Provide a CONFIG_DM_STDIO option to enable registering a serial device
with the stdio library. This is seldom useful in SPL, so disable it by
default when building for SPL.

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

include/config_defaults.h

@@ -22,6 +22,7 @@
 
 #ifndef CONFIG_SPL_BUILD
 #define CONFIG_DM_DEVICE_REMOVE
+#define CONFIG_DM_STDIO
 #endif
 
 #endif