[POWERPC] bootwrapper: Use map_string() instead of lookup_string() in ft_prop().

When adding a property, the property name should be added to the string
table if it doesn't already exist.  map_string() does that;
lookup_string() will fail instead.
Signed-off-by: Scott Wood <scottwood@freescale.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/boot/flatdevtree.c

@@ -429,7 +429,7 @@ int ft_prop(struct ft_cxt *cxt, const char *name, const void *data,
 {
 	int off, len;
 
-	off = lookup_string(cxt, name);
+	off = map_string(cxt, name);
 	if (off == NO_STRING)
 		return -1;