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/i2c/tegra186_bpmp_i2c.c b/drivers/i2c/tegra186_bpmp_i2c.c
index 513c414..931c6de 100644
--- a/drivers/i2c/tegra186_bpmp_i2c.c
+++ b/drivers/i2c/tegra186_bpmp_i2c.c
@@ -90,7 +90,7 @@
 {
 	struct tegra186_bpmp_i2c *priv = dev_get_priv(dev);
 
-	priv->bpmp_bus_id = fdtdec_get_uint(gd->fdt_blob, dev->of_offset,
+	priv->bpmp_bus_id = fdtdec_get_uint(gd->fdt_blob, dev_of_offset(dev),
 					    "nvidia,bpmp-bus-id", U32_MAX);
 	if (priv->bpmp_bus_id == U32_MAX) {
 		debug("%s: could not parse nvidia,bpmp-bus-id\n", __func__);