dm: core: Replace of_offset with accessor (part 2)

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/zynq_sdhci.c b/drivers/mmc/zynq_sdhci.c
index 70ad573..0fddb42 100644
--- a/drivers/mmc/zynq_sdhci.c
+++ b/drivers/mmc/zynq_sdhci.c
@@ -83,7 +83,7 @@
 	host->name = dev->name;
 	host->ioaddr = (void *)devfdt_get_addr(dev);
 
-	plat->f_max = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	plat->f_max = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 				"max-frequency", CONFIG_ZYNQ_SDHCI_MAX_FREQ);
 
 	return 0;