dm: core: Replace of_offset with accessor (part 2)

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/serial/serial_lpuart.c b/drivers/serial/serial_lpuart.c
index 47dd88b..382f8ba 100644
--- a/drivers/serial/serial_lpuart.c
+++ b/drivers/serial/serial_lpuart.c
@@ -416,7 +416,7 @@
 {
 	struct lpuart_serial_platdata *plat = dev->platdata;
 	const void *blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	fdt_addr_t addr;
 
 	addr = devfdt_get_addr(dev);