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/serial/serial_pic32.c b/drivers/serial/serial_pic32.c
index c2141f0..b0e01aa 100644
--- a/drivers/serial/serial_pic32.c
+++ b/drivers/serial/serial_pic32.c
@@ -141,7 +141,8 @@
 	int ret;
 
 	/* get address */
-	addr = fdtdec_get_addr_size(gd->fdt_blob, dev->of_offset, "reg", &size);
+	addr = fdtdec_get_addr_size(gd->fdt_blob, dev_of_offset(dev), "reg",
+				    &size);
 	if (addr == FDT_ADDR_T_NONE)
 		return -EINVAL;