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/tegra.c b/drivers/video/tegra.c
index 92214d6..0ba3f2c 100644
--- a/drivers/video/tegra.c
+++ b/drivers/video/tegra.c
@@ -338,7 +338,7 @@
 	struct tegra_lcd_priv *priv = dev_get_priv(dev);
 	const void *blob = gd->fdt_blob;
 	struct display_timing *timing;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	int panel_node;
 	int rgb;
 	int ret;
@@ -392,7 +392,7 @@
 {
 	struct video_uc_platdata *plat = dev_get_uclass_platdata(dev);
 	const void *blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	int rgb;
 
 	rgb = fdt_subnode_offset(blob, node, "rgb");