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/video/bridge/ps862x.c b/drivers/video/bridge/ps862x.c
index 80f63e3..46aacca 100644
--- a/drivers/video/bridge/ps862x.c
+++ b/drivers/video/bridge/ps862x.c
@@ -86,7 +86,8 @@
 	if (ret)
 		return ret;
 
-	params = fdt_getprop(gd->fdt_blob, dev->of_offset, "parade,regs", &len);
+	params = fdt_getprop(gd->fdt_blob, dev_of_offset(dev), "parade,regs",
+			     &len);
 	if (!params || len % 3) {
 		debug("%s: missing/invalid params=%p, len=%x\n", __func__,
 		      params, len);