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_mxc.c b/drivers/serial/serial_mxc.c
index 4fd2b1d..1cfcbf2 100644
--- a/drivers/serial/serial_mxc.c
+++ b/drivers/serial/serial_mxc.c
@@ -359,7 +359,7 @@
 
 	plat->reg = (struct mxc_uart *)addr;
 
-	plat->use_dte = fdtdec_get_bool(gd->fdt_blob, dev->of_offset,
+	plat->use_dte = fdtdec_get_bool(gd->fdt_blob, dev_of_offset(dev),
 					"fsl,dte-mode");
 	return 0;
 }