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>

include/dm/device.h

@@ -121,6 +121,16 @@ struct udevice {
 /* Returns non-zero if the device is active (probed and not removed) */
 #define device_active(dev)	((dev)->flags & DM_FLAG_ACTIVATED)
 
+static inline int dev_of_offset(const struct udevice *dev)
+{
+	return dev->of_offset;
+}
+
+static inline void dev_set_of_offset(struct udevice *dev, int of_offset)
+{
+	dev->of_offset = of_offset;
+}
+
 /**
  * struct udevice_id - Lists the compatible strings supported by a driver
  * @compatible: Compatible string