of: Improve prom_update_property() function

prom_update_property() currently fails if the property doesn't
actually exist yet which isn't what we want. Change to add-or-update
instead of update-only, then we can remove a lot duplicated lines.

Suggested-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Dong Aisheng <dong.aisheng@linaro.org>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

fs/proc/proc_devtree.c

@@ -101,6 +101,11 @@ void proc_device_tree_update_prop(struct proc_dir_entry *pde,
 {
 	struct proc_dir_entry *ent;
 
+	if (!oldprop) {
+		proc_device_tree_add_prop(pde, newprop);
+		return;
+	}
+
 	for (ent = pde->subdir; ent != NULL; ent = ent->next)
 		if (ent->data == oldprop)
 			break;