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/demo/demo-shape.c b/drivers/demo/demo-shape.c
index d908736..7e71f3b 100644
--- a/drivers/demo/demo-shape.c
+++ b/drivers/demo/demo-shape.c
@@ -151,7 +151,7 @@
 		return ret;
 
 	/* Parse the data that only we need */
-	pdata->default_char = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	pdata->default_char = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 					     "character", '@');
 
 	return 0;