sandbox: serial: Convert to livetree

Use a livetree function to read the colour.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/sandbox.c b/drivers/serial/sandbox.c
index e3967de..8c74c3a 100644
--- a/drivers/serial/sandbox.c
+++ b/drivers/serial/sandbox.c
@@ -12,7 +12,6 @@
 #include <common.h>
 #include <console.h>
 #include <dm.h>
-#include <fdtdec.h>
 #include <lcd.h>
 #include <os.h>
 #include <serial.h>
@@ -221,8 +220,7 @@
 	if (CONFIG_IS_ENABLED(OF_PLATDATA))
 		return 0;
 	plat->colour = -1;
-	colour = fdt_getprop(gd->fdt_blob, dev_of_offset(dev),
-			     "sandbox,text-colour", NULL);
+	colour = dev_read_string(dev, "sandbox,text-colour");
 	if (colour) {
 		for (i = 0; i < ARRAY_SIZE(ansi_colour); i++) {
 			if (!strcmp(colour, ansi_colour[i])) {