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/i8042.c b/drivers/input/i8042.c
index 03d4840..0fd25b1 100644
--- a/drivers/input/i8042.c
+++ b/drivers/input/i8042.c
@@ -315,7 +315,7 @@
 	struct input_config *input = &uc_priv->input;
 	int ret;
 
-	if (fdtdec_get_bool(gd->fdt_blob, dev->of_offset,
+	if (fdtdec_get_bool(gd->fdt_blob, dev_of_offset(dev),
 			    "intel,duplicate-por"))
 		priv->quirks |= QUIRK_DUP_POR;