Fix ambiguous grammar for devicetree rule

Commit 737b2df3, "overlay: Add syntactic sugar version of overlays"
introduced an empty rule for "devicetree" that created ambiguities in
the grammar and causes the following warning:

         BISON dtc-parser.tab.c
  dtc-parser.y: warning: 3 shift/reduce conflicts [-Wconflicts-sr]

Fix the grammar by explicitly testing for the condition the
new overlay grammar wants to use. This means duplicating a very small
amount of grammar processing code, but the alternative seems to be a
more invasive reorganization of the devicetree rule. Better to fix it
this way now and save the reorg for a separate patch.

Signed-off-by: Grant Likely <grant.likely@arm.com>
Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Cc: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtc-parser.y

@@ -166,7 +166,17 @@ devicetree:
 		{
 			$$ = merge_nodes($1, $3);
 		}
-
+	| DT_REF nodedef
+		{
+			/*
+			 * We rely on the rule being always:
+			 *   versioninfo plugindecl memreserves devicetree
+			 * so $-1 is what we want (plugindecl)
+			 */
+			if (!($<flags>-1 & DTSF_PLUGIN))
+				ERROR(&@2, "Label or path %s not found", $1);
+			$$ = add_orphan_node(name_node(build_node(NULL, NULL), ""), $2, $1);
+		}
 	| devicetree DT_LABEL DT_REF nodedef
 		{
 			struct node *target = get_node_by_ref($1, $3);
@@ -209,11 +219,6 @@ devicetree:
 
 			$$ = $1;
 		}
-	| /* empty */
-		{
-			/* build empty node */
-			$$ = name_node(build_node(NULL, NULL), "");
-		}
 	;
 
 nodedef:

dtc.h

@@ -204,7 +204,7 @@ struct node *build_node_delete(void);
 struct node *name_node(struct node *node, char *name);
 struct node *chain_node(struct node *first, struct node *list);
 struct node *merge_nodes(struct node *old_node, struct node *new_node);
-void add_orphan_node(struct node *old_node, struct node *new_node, char *ref);
+struct node *add_orphan_node(struct node *old_node, struct node *new_node, char *ref);
 
 void add_property(struct node *node, struct property *prop);
 void delete_property_by_name(struct node *node, char *name);

livetree.c

@@ -216,7 +216,7 @@ struct node *merge_nodes(struct node *old_node, struct node *new_node)
 	return old_node;
 }
 
-void add_orphan_node(struct node *dt, struct node *new_node, char *ref)
+struct node * add_orphan_node(struct node *dt, struct node *new_node, char *ref)
 {
 	static unsigned int next_orphan_fragment = 0;
 	struct node *node;
@@ -236,6 +236,7 @@ void add_orphan_node(struct node *dt, struct node *new_node, char *ref)
 	name_node(node, name);
 
 	add_child(dt, node);
+	return dt;
 }
 
 struct node *chain_node(struct node *first, struct node *list)