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/i2c/i2c-gpio.c b/drivers/i2c/i2c-gpio.c
index a8b83c5..aeeb304 100644
--- a/drivers/i2c/i2c-gpio.c
+++ b/drivers/i2c/i2c-gpio.c
@@ -309,7 +309,7 @@
 {
 	struct i2c_gpio_bus *bus = dev_get_priv(dev);
 	const void *blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	int ret;
 
 	ret = gpio_request_list_by_name(dev, "gpios", bus->gpios,