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/input/cros_ec_keyb.c b/drivers/input/cros_ec_keyb.c
index 9bc4555..00381dc 100644
--- a/drivers/input/cros_ec_keyb.c
+++ b/drivers/input/cros_ec_keyb.c
@@ -189,7 +189,7 @@
 	struct stdio_dev *sdev = &uc_priv->sdev;
 	struct input_config *input = &uc_priv->input;
 	const void *blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	int ret;
 
 	if (cros_ec_keyb_decode_fdt(blob, node, priv))