dm: core: Allow dm_warn() to be used in SPL

At present this option is disabled in SPL, meaning that warnings are not
displayed. It is sometimes useful to see warnings in SPL for debugging
purposes.

Add a new Kconfig option to permit this.

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

drivers/core/util.c

@@ -11,7 +11,7 @@
 #include <linux/libfdt.h>
 #include <vsprintf.h>
 
-#ifdef CONFIG_DM_WARN
+#if CONFIG_IS_ENABLED(DM_WARN)
 void dm_warn(const char *fmt, ...)
 {
 	va_list args;