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/mtd/pic32_flash.c b/drivers/mtd/pic32_flash.c
index 9166fcd..8ed7874 100644
--- a/drivers/mtd/pic32_flash.c
+++ b/drivers/mtd/pic32_flash.c
@@ -371,7 +371,7 @@
 static int pic32_flash_probe(struct udevice *dev)
 {
 	void *blob = (void *)gd->fdt_blob;
-	int node = dev->of_offset;
+	int node = dev_of_offset(dev);
 	const char *list, *end;
 	const fdt32_t *cell;
 	unsigned long addr, size;