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/mvebu_a3700_spi.c b/drivers/spi/mvebu_a3700_spi.c
index 7c58c36..ee847e4 100644
--- a/drivers/spi/mvebu_a3700_spi.c
+++ b/drivers/spi/mvebu_a3700_spi.c
@@ -261,9 +261,9 @@
 	 * it should be used to read the input clock and the DT property
 	 * can be removed.
 	 */
-	plat->clock = fdtdec_get_int(gd->fdt_blob, bus->of_offset,
+	plat->clock = fdtdec_get_int(gd->fdt_blob, dev_of_offset(bus),
 				     "clock-frequency", 160000);
-	plat->frequency = fdtdec_get_int(gd->fdt_blob, bus->of_offset,
+	plat->frequency = fdtdec_get_int(gd->fdt_blob, dev_of_offset(bus),
 					 "spi-max-frequency", 40000);
 
 	return 0;