dm: core: Replace of_offset with accessor

At present devices use a simple integer offset to record the device tree
node associated with the device. In preparation for supporting a live
device tree, which uses a node pointer instead, refactor existing code to
access this field through an inline function.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/sandbox.c b/drivers/serial/sandbox.c
index bcc3465..858e340 100644
--- a/drivers/serial/sandbox.c
+++ b/drivers/serial/sandbox.c
@@ -152,7 +152,7 @@
 	int i;
 
 	plat->colour = -1;
-	colour = fdt_getprop(gd->fdt_blob, dev->of_offset,
+	colour = fdt_getprop(gd->fdt_blob, dev_of_offset(dev),
 			     "sandbox,text-colour", NULL);
 	if (colour) {
 		for (i = 0; i < ARRAY_SIZE(ansi_colour); i++) {