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/arch/nios2/cpu/cpu.c b/arch/nios2/cpu/cpu.c
index 4f0f8fc..c1caa65 100644
--- a/arch/nios2/cpu/cpu.c
+++ b/arch/nios2/cpu/cpu.c
@@ -103,7 +103,7 @@
 static int altera_nios2_probe(struct udevice *dev)
 {
 	const void *blob = gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 
 	gd->cpu_clk = fdtdec_get_int(blob, node,
 		"clock-frequency", 0);