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_uniphier.c b/drivers/serial/serial_uniphier.c
index 4ea5304..0e25cba 100644
--- a/drivers/serial/serial_uniphier.c
+++ b/drivers/serial/serial_uniphier.c
@@ -105,7 +105,7 @@
 
 	priv->membase = port;
 
-	priv->uartclk = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	priv->uartclk = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 				       "clock-frequency", 0);
 
 	tmp = readl(&port->lcr_mcr);