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/pwm_backlight.c b/drivers/video/pwm_backlight.c
index de6277f..3697f49 100644
--- a/drivers/video/pwm_backlight.c
+++ b/drivers/video/pwm_backlight.c
@@ -61,7 +61,7 @@
 	struct pwm_backlight_priv *priv = dev_get_priv(dev);
 	struct fdtdec_phandle_args args;
 	const void *blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	int index, ret, count, len;
 	const u32 *cell;