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/net/phy/xilinx_phy.c b/drivers/net/phy/xilinx_phy.c
index 920bfcb..3f80f04 100644
--- a/drivers/net/phy/xilinx_phy.c
+++ b/drivers/net/phy/xilinx_phy.c
@@ -104,7 +104,7 @@
 	u32 phytype;
 
 	debug("%s\n", __func__);
-	phytype = fdtdec_get_int(gd->fdt_blob, phydev->dev->of_offset,
+	phytype = fdtdec_get_int(gd->fdt_blob, dev_of_offset(phydev->dev),
 				 "phy-type", -1);
 	if (phytype == XAE_PHY_TYPE_1000BASE_X)
 		phydev->flags |= XAE_PHY_TYPE_1000BASE_X;