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/atmel_lcdfb.c b/drivers/video/atmel_lcdfb.c
index 39cd7ca..ed2bd30 100644
--- a/drivers/video/atmel_lcdfb.c
+++ b/drivers/video/atmel_lcdfb.c
@@ -276,7 +276,7 @@
 	struct display_timing *timing = &priv->timing;
 	const void *blob = gd->fdt_blob;
 
-	if (fdtdec_decode_display_timing(blob, dev->of_offset,
+	if (fdtdec_decode_display_timing(blob, dev_of_offset(dev),
 					 plat->timing_index, timing)) {
 		debug("%s: Failed to decode display timing\n", __func__);
 		return -EINVAL;