dtc: Generate useful error message for properties after subnodes

On several occasions, I've accidentally put properties after subnodes
in a dts file.  I've then spent ages thinking that the resulting
syntax error was because of something else.

This patch arranges for this specific syntax error to generate a more
specific and useful error message.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

dtc-parser.y

@@ -276,6 +276,11 @@ subnodes:
 		{
 			$$ = chain_node($1, $2);
 		}
+	| subnode propdef
+		{
+			yyerror("syntax error: properties must precede subnodes\n");
+			YYERROR;
+		}
 	;
 
 subnode: