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/pci/pcie_xilinx.c b/drivers/pci/pcie_xilinx.c
index 5216001..08e2e93 100644
--- a/drivers/pci/pcie_xilinx.c
+++ b/drivers/pci/pcie_xilinx.c
@@ -186,7 +186,7 @@
 	DECLARE_GLOBAL_DATA_PTR;
 	int err;
 
-	err = fdt_get_resource(gd->fdt_blob, dev->of_offset, "reg",
+	err = fdt_get_resource(gd->fdt_blob, dev_of_offset(dev), "reg",
 			       0, &reg_res);
 	if (err < 0) {
 		error("\"reg\" resource not found\n");