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/tps65090.c b/drivers/power/pmic/tps65090.c
index 4797f32..b30a7f0 100644
--- a/drivers/power/pmic/tps65090.c
+++ b/drivers/power/pmic/tps65090.c
@@ -56,7 +56,7 @@
 	const void *blob = gd->fdt_blob;
 	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__,