test: Detect a change in the device tree

If the device tree changes during a test and we cannot restore it, mark
it as such so that future tests which need the live tree are skipped.

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

include/asm-generic/global_data.h

@@ -646,6 +646,10 @@ enum gd_flags {
 	 * @GD_FLG_SMP_READY: SMP initialization is complete
 	 */
 	GD_FLG_SMP_READY = 0x80000,
+	/**
+	 * @GD_FLG_FDT_CHANGED: Device tree change has been detected by tests
+	 */
+	GD_FLG_FDT_CHANGED = 0x100000,
 };
 
 #endif /* __ASSEMBLY__ */