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/arch/arm/mach-aspeed/ast2500/sdram_ast2500.c b/arch/arm/mach-aspeed/ast2500/sdram_ast2500.c
index ace1028..cb6e03f 100644
--- a/arch/arm/mach-aspeed/ast2500/sdram_ast2500.c
+++ b/arch/arm/mach-aspeed/ast2500/sdram_ast2500.c
@@ -392,7 +392,7 @@
 	priv->regs = regmap_get_range(map, 0);
 	priv->phy = regmap_get_range(map, 1);
 
-	priv->clock_rate = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	priv->clock_rate = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 					  "clock-frequency", 0);
 
 	if (!priv->clock_rate) {