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/ivybridge_igd.c b/drivers/video/ivybridge_igd.c
index f16d725..d8af2e1 100644
--- a/drivers/video/ivybridge_igd.c
+++ b/drivers/video/ivybridge_igd.c
@@ -517,7 +517,7 @@
 static int gma_pm_init_post_vbios(struct udevice *dev, int rev, void *gtt_bar)
 {
 	const void *blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	u32 reg32, cycle_delay;
 
 	debug("GT Power Management Init (post VBIOS)\n");