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/video/rockchip/rk_vop.c b/drivers/video/rockchip/rk_vop.c
index eab5486..aeecb58 100644
--- a/drivers/video/rockchip/rk_vop.c
+++ b/drivers/video/rockchip/rk_vop.c
@@ -321,7 +321,7 @@
 	 * clock so it is currently not possible to use more than one display
 	 * device simultaneously.
 	 */
-	port = fdt_subnode_offset(blob, dev->of_offset, "port");
+	port = fdt_subnode_offset(blob, dev_of_offset(dev), "port");
 	if (port < 0)
 		return -EINVAL;
 	for (node = fdt_first_subnode(blob, port);