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/spi/fsl_dspi.c b/drivers/spi/fsl_dspi.c
index f213587..e09985e 100644
--- a/drivers/spi/fsl_dspi.c
+++ b/drivers/spi/fsl_dspi.c
@@ -646,7 +646,7 @@
 	fdt_addr_t addr;
 	struct fsl_dspi_platdata *plat = bus->platdata;
 	const void *blob = gd->fdt_blob;
-	int node = bus->of_offset;
+	int node = dev_of_offset(bus);
 
 	if (fdtdec_get_bool(blob, node, "big-endian"))
 		plat->flags |= DSPI_FLAG_REGMAP_ENDIAN_BIG;