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/gpio/altera_pio.c b/drivers/gpio/altera_pio.c
index 6f42bf8..92849c5 100644
--- a/drivers/gpio/altera_pio.c
+++ b/drivers/gpio/altera_pio.c
@@ -92,9 +92,9 @@
 	plat->regs = map_physmem(dev_get_addr(dev),
 				 sizeof(struct altera_pio_regs),
 				 MAP_NOCACHE);
-	plat->gpio_count = fdtdec_get_int(gd->fdt_blob, dev->of_offset,
+	plat->gpio_count = fdtdec_get_int(gd->fdt_blob, dev_of_offset(dev),
 		"altr,gpio-bank-width", 32);
-	plat->bank_name = fdt_getprop(gd->fdt_blob, dev->of_offset,
+	plat->bank_name = fdt_getprop(gd->fdt_blob, dev_of_offset(dev),
 		"gpio-bank-name", NULL);
 
 	return 0;