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/gpio/intel_ich6_gpio.c b/drivers/gpio/intel_ich6_gpio.c
index 0a9eb03..ffc3ccb 100644
--- a/drivers/gpio/intel_ich6_gpio.c
+++ b/drivers/gpio/intel_ich6_gpio.c
@@ -129,7 +129,7 @@
 	bank->io_sel = plat->base_addr + 4;
 	bank->lvl = plat->base_addr + 8;
 
-	prop = fdt_getprop(gd->fdt_blob, dev->of_offset,
+	prop = fdt_getprop(gd->fdt_blob, dev_of_offset(dev),
 			   "use-lvl-write-cache", NULL);
 	if (prop)
 		bank->use_lvl_write_cache = true;