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/serial/serial_pl01x.c b/drivers/serial/serial_pl01x.c
index a8d3d67..a49134a 100644
--- a/drivers/serial/serial_pl01x.c
+++ b/drivers/serial/serial_pl01x.c
@@ -354,9 +354,10 @@
 		return -EINVAL;
 
 	plat->base = addr;
-	plat->clock = fdtdec_get_int(gd->fdt_blob, dev->of_offset, "clock", 1);
+	plat->clock = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev), "clock",
+				     1);
 	plat->type = dev_get_driver_data(dev);
-	plat->skip_init = fdtdec_get_bool(gd->fdt_blob, dev->of_offset,
+	plat->skip_init = fdtdec_get_bool(gd->fdt_blob, dev_of_offset(dev),
 	                                  "skip-init");
 	return 0;
 }