libdt: add of_property_write_string() ...

This ports the following barebox commits:

| commit 2d36433
| Author: Sascha Hauer <s.hauer@pengutronix.de>
| Date:   Thu Mar 30 10:36:58 2017 +0200
|
|     of: Add of_property_write_string()
|
|     Setting a property to a string is used many times. Create a convenience
|     function for it.
|
|     Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

| commit 4da7cf0
| Author: Ulrich Ölmann <u.oelmann@pengutronix.de>
| Date:   Fri Oct 19 06:33:11 2018 +0200
|
|     of: base: fix typo
|
|     Signed-off-by: Ulrich Ölmann <u.oelmann@pengutronix.de>
|     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>

barebox-state: Import updated state code ...

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>

barebox-state: import from barebox ...

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

libdt: import from barebox-2015.04.0 ...

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>

state: describe backend with phandle ...

We used to describe the backend with a combination of a nodepath
and partname:<name>. This is cumbersome and error prone to parse.
Instead, describe the backend with a phandle that directly points
to the partition. The resulting code is easier to read and the
devicetrees are easier to understand. Tested with both mtd and
EEPROM device, both with partitions with an offset != 0.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

2nd commit ...

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>