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/sandbox.c b/drivers/power/pmic/sandbox.c
index 3e56acd..b4e412e 100644
--- a/drivers/power/pmic/sandbox.c
+++ b/drivers/power/pmic/sandbox.c
@@ -51,7 +51,7 @@
 
 static int sandbox_pmic_bind(struct udevice *dev)
 {
-	if (!pmic_bind_children(dev, dev->of_offset, pmic_children_info))
+	if (!pmic_bind_children(dev, dev_of_offset(dev), pmic_children_info))
 		error("%s:%d PMIC: %s - no child found!", __func__, __LINE__,
 							  dev->name);