dm: Avoid using #ifdef for CONFIG_OF_LIVE

At present this option results in a number of #ifdefs due to the presence
or absence of the global_data of_root member.

Add a few macros to global_data.h to work around this. Update the code
accordingly.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/dm/of.h b/include/dm/of.h
index 6bef73b..5cb6f44 100644
--- a/include/dm/of.h
+++ b/include/dm/of.h
@@ -90,17 +90,10 @@
  *
  * @returns true if livetree is active, false it not
  */
-#ifdef CONFIG_OF_LIVE
 static inline bool of_live_active(void)
 {
-	return gd->of_root != NULL;
-}
-#else
-static inline bool of_live_active(void)
-{
-	return false;
+	return gd_of_root() != NULL;
 }
-#endif
 
 #define OF_BAD_ADDR	((u64)-1)