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/at91/clk-generated.c b/drivers/clk/at91/clk-generated.c
index d36f64f..ac27d3e 100644
--- a/drivers/clk/at91/clk-generated.c
+++ b/drivers/clk/at91/clk-generated.c
@@ -154,9 +154,8 @@
 	u32 num_parents;
 
 	num_parents = fdtdec_get_int_array_count(gd->fdt_blob,
-						 dev_get_parent(dev)->of_offset,
-						 "clocks", cells,
-						 GENERATED_SOURCE_MAX);
+			dev_of_offset(dev_get_parent(dev)), "clocks", cells,
+			GENERATED_SOURCE_MAX);
 
 	if (!num_parents)
 		return -1;