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/palmas.c b/drivers/power/pmic/palmas.c
index 0ab425e..f5a2307 100644
--- a/drivers/power/pmic/palmas.c
+++ b/drivers/power/pmic/palmas.c
@@ -49,7 +49,7 @@
 	int pmic_node = -1, regulators_node;
 	const void *blob = gd->fdt_blob;
 	int children;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	int subnode, len;
 
 	fdt_for_each_subnode(subnode, blob, node) {