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/mpc85xx_gpio.c b/drivers/gpio/mpc85xx_gpio.c
index 168c696..cfeb6e7 100644
--- a/drivers/gpio/mpc85xx_gpio.c
+++ b/drivers/gpio/mpc85xx_gpio.c
@@ -169,13 +169,13 @@
 	fdt_addr_t addr;
 	fdt_size_t size;
 
-	addr = fdtdec_get_addr_size_auto_noparent(gd->fdt_blob, dev->of_offset,
-						  "reg", 0, &size, false);
+	addr = fdtdec_get_addr_size_auto_noparent(gd->fdt_blob,
+			dev_of_offset(dev), "reg", 0, &size, false);
 
 	plat->addr = addr;
 	plat->size = size;
-	plat->ngpios = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
-						  "ngpios", 32);
+	plat->ngpios = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
+				      "ngpios", 32);
 
 	return 0;
 }