This ports the following barebox commit:

| commit b608918
| Author: Sascha Hauer <s.hauer@pengutronix.de>
| Date:   Thu Mar 30 10:38:19 2017 +0200
|
|     treewide: Use of_property_write_string() where appropriate
|
|     Replace users which use of_set_property() to set a property to a string
|     with of_property_write_string().
|
|     Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Signed-off-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
Signed-off-by: Roland Hieber <rhi@pengutronix.de>