dm: core: Add an ofnode function to get the devicetree root

This is needed in at least one place. Avoid the conditional code in root.c
by adding this inline function.

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

include/dm/ofnode.h

@@ -218,6 +218,18 @@ static inline ofnode ofnode_null(void)
 	return node;
 }
 
+static inline ofnode ofnode_root(void)
+{
+	ofnode node;
+
+	if (of_live_active())
+		node.np = gd_of_root();
+	else
+		node.of_offset = 0;
+
+	return node;
+}
+
 /**
  * ofnode_read_u32() - Read a 32-bit integer from a property
  *