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/x86/cpu/irq.c b/arch/x86/cpu/irq.c
index d2901de..f5654eb 100644
--- a/arch/x86/cpu/irq.c
+++ b/arch/x86/cpu/irq.c
@@ -96,7 +96,7 @@
 	int i;
 	int ret;
 
-	node = dev->of_offset;
+	node = dev_of_offset(dev);
 
 	/* extract the bdf from fdt_pci_addr */
 	priv->bdf = dm_pci_get_bdf(dev->parent);