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/led/led_gpio.c b/drivers/led/led_gpio.c
index cb6e996..5b11990 100644
--- a/drivers/led/led_gpio.c
+++ b/drivers/led/led_gpio.c
@@ -62,7 +62,7 @@
 	int node;
 	int ret;
 
-	for (node = fdt_first_subnode(blob, parent->of_offset);
+	for (node = fdt_first_subnode(blob, dev_of_offset(parent));
 	     node > 0;
 	     node = fdt_next_subnode(blob, node)) {
 		struct led_uclass_plat *uc_plat;