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/power/pmic/act8846.c b/drivers/power/pmic/act8846.c
index e8164bf..15da12e 100644
--- a/drivers/power/pmic/act8846.c
+++ b/drivers/power/pmic/act8846.c
@@ -52,7 +52,7 @@
 	int regulators_node;
 	int children;
 
-	regulators_node = fdt_subnode_offset(blob, dev->of_offset,
+	regulators_node = fdt_subnode_offset(blob, dev_of_offset(dev),
 					     "regulators");
 	if (regulators_node <= 0) {
 		debug("%s: %s regulators subnode not found!", __func__,