dm: core: Replace of_offset with accessor (part 2)

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/clk/clk_zynq.c b/drivers/clk/clk_zynq.c
index 6edc4dc..b997545 100644
--- a/drivers/clk/clk_zynq.c
+++ b/drivers/clk/clk_zynq.c
@@ -466,7 +466,7 @@
 	}
 #endif
 
-	priv->ps_clk_freq = fdtdec_get_uint(gd->fdt_blob, dev->of_offset,
+	priv->ps_clk_freq = fdtdec_get_uint(gd->fdt_blob, dev_of_offset(dev),
 					    "ps-clk-frequency", 33333333UL);
 
 	return 0;