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/gpio/sunxi_gpio.c b/drivers/gpio/sunxi_gpio.c
index e8accaa..8d2bb18 100644
--- a/drivers/gpio/sunxi_gpio.c
+++ b/drivers/gpio/sunxi_gpio.c
@@ -312,7 +312,7 @@
 					plat->bank_name, plat, -1, &dev);
 		if (ret)
 			return ret;
-		dev->of_offset = parent->of_offset;
+		dev_set_of_offset(dev, dev_of_offset(parent));
 	}
 
 	return 0;