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/firmware/psci.c b/drivers/firmware/psci.c
index 3ae627c..7652cc2 100644
--- a/drivers/firmware/psci.c
+++ b/drivers/firmware/psci.c
@@ -59,8 +59,8 @@
 	DECLARE_GLOBAL_DATA_PTR;
 	const char *method;
 
-	method = fdt_stringlist_get(gd->fdt_blob, dev->of_offset, "method", 0,
-				    NULL);
+	method = fdt_stringlist_get(gd->fdt_blob, dev_of_offset(dev), "method",
+				    0, NULL);
 	if (!method) {
 		printf("missing \"method\" property\n");
 		return -ENXIO;