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/omap_gpio.c b/drivers/gpio/omap_gpio.c
index f906b97..5338552 100644
--- a/drivers/gpio/omap_gpio.c
+++ b/drivers/gpio/omap_gpio.c
@@ -320,7 +320,7 @@
 		return -ENOMEM;
 
 	plat->base = base_addr;
-	plat->port_name = fdt_get_name(gd->fdt_blob, dev->of_offset, NULL);
+	plat->port_name = fdt_get_name(gd->fdt_blob, dev_of_offset(dev), NULL);
 	dev->platdata = plat;
 
 	return 0;