dm: core: Replace of_offset with accessor (part 2)

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/timer/arc_timer.c b/drivers/timer/arc_timer.c
index e94e4a4..a5f6b34 100644
--- a/drivers/timer/arc_timer.c
+++ b/drivers/timer/arc_timer.c
@@ -51,7 +51,7 @@
 	struct arc_timer_priv *priv = dev_get_priv(dev);
 
 	/* Get registers offset and size */
-	id = fdtdec_get_int(gd->fdt_blob, dev->of_offset, "reg", -1);
+	id = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev), "reg", -1);
 	if (id < 0)
 		return -EINVAL;