dm: core: Prepare for updating the device tree with ofnode

Add some documentation and a new flag so that we can safely enabled using
the ofnode interface to write to the device tree.

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

include/test/test.h

@@ -46,6 +46,8 @@ enum {
 	UT_TESTF_CONSOLE_REC	= BIT(5),	/* needs console recording */
 	/* do extra driver model init and uninit */
 	UT_TESTF_DM		= BIT(6),
+	/* live or flat device tree, but not both in the same executable */
+	UT_TESTF_LIVE_OR_FLAT	= BIT(4),
 };
 
 /**