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/pca953x_gpio.c b/drivers/gpio/pca953x_gpio.c
index 0410add..b81f0fa 100644
--- a/drivers/gpio/pca953x_gpio.c
+++ b/drivers/gpio/pca953x_gpio.c
@@ -265,7 +265,7 @@
 		return -ENODEV;
 	}
 
-	addr = fdtdec_get_int(gd->fdt_blob, dev->of_offset, "reg", 0);
+	addr = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev), "reg", 0);
 	if (addr == 0)
 		return -ENODEV;