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_lvds.c b/drivers/video/rockchip/rk_lvds.c
index fcbb4d6..ee43255 100644
--- a/drivers/video/rockchip/rk_lvds.c
+++ b/drivers/video/rockchip/rk_lvds.c
@@ -162,7 +162,7 @@
 int rk_lvds_read_timing(struct udevice *dev, struct display_timing *timing)
 {
 	if (fdtdec_decode_display_timing
-	    (gd->fdt_blob, dev->of_offset, 0, timing)) {
+	    (gd->fdt_blob, dev_of_offset(dev), 0, timing)) {
 		debug("%s: Failed to decode display timing\n", __func__);
 		return -EINVAL;
 	}
@@ -174,7 +174,7 @@
 {
 	struct rk_lvds_priv *priv = dev_get_priv(dev);
 	const void *blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	int ret;
 	priv->regs = (void *)dev_get_addr(dev);
 	priv->grf = syscon_get_first_range(ROCKCHIP_SYSCON_GRF);