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/clk/clk_fixed_rate.c b/drivers/clk/clk_fixed_rate.c
index 9c4d2b3..3911bf6 100644
--- a/drivers/clk/clk_fixed_rate.c
+++ b/drivers/clk/clk_fixed_rate.c
@@ -32,7 +32,7 @@
 {
 #if !CONFIG_IS_ENABLED(OF_PLATDATA)
 	to_clk_fixed_rate(dev)->fixed_rate =
-				fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+				fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 					       "clock-frequency", 0);
 #endif