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/sandbox.c b/drivers/gpio/sandbox.c
index f6435a0..ae6d930 100644
--- a/drivers/gpio/sandbox.c
+++ b/drivers/gpio/sandbox.c
@@ -197,9 +197,9 @@
 {
 	struct gpio_dev_priv *uc_priv = dev_get_uclass_priv(dev);
 
-	uc_priv->gpio_count = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	uc_priv->gpio_count = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 					     "num-gpios", 0);
-	uc_priv->bank_name = fdt_getprop(gd->fdt_blob, dev->of_offset,
+	uc_priv->bank_name = fdt_getprop(gd->fdt_blob, dev_of_offset(dev),
 					 "gpio-bank-name", NULL);
 
 	return 0;
@@ -209,7 +209,7 @@
 {
 	struct gpio_dev_priv *uc_priv = dev_get_uclass_priv(dev);
 
-	if (dev->of_offset == -1) {
+	if (dev_of_offset(dev) == -1) {
 		/* Tell the uclass how many GPIOs we have */
 		uc_priv->gpio_count = CONFIG_SANDBOX_GPIO_COUNT;
 	}