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/mmc/rockchip_sdhci.c b/drivers/mmc/rockchip_sdhci.c
index 1358864..bd91f91 100644
--- a/drivers/mmc/rockchip_sdhci.c
+++ b/drivers/mmc/rockchip_sdhci.c
@@ -38,7 +38,7 @@
 	struct clk clk;
 
 
-	max_frequency = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	max_frequency = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 			"max-frequency", 0);
 	ret = clk_get_by_index(dev, 0, &clk);
 	if (!ret) {