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/x86/cpu/ivybridge/lpc.c b/arch/x86/cpu/ivybridge/lpc.c
index aef1206..4e254b3 100644
--- a/arch/x86/cpu/ivybridge/lpc.c
+++ b/arch/x86/cpu/ivybridge/lpc.c
@@ -86,7 +86,7 @@
 {
 	uint8_t route[8], *ptr;
 
-	if (fdtdec_get_byte_array(gd->fdt_blob, pch->of_offset,
+	if (fdtdec_get_byte_array(gd->fdt_blob, dev_of_offset(pch),
 				  "intel,pirq-routing", route, sizeof(route)))
 		return -EINVAL;
 	ptr = route;
@@ -113,7 +113,7 @@
 	u32 reg;
 	int gpi;
 
-	if (fdtdec_get_byte_array(gd->fdt_blob, pch->of_offset,
+	if (fdtdec_get_byte_array(gd->fdt_blob, dev_of_offset(pch),
 				  "intel,gpi-routing", route, sizeof(route)))
 		return -EINVAL;
 
@@ -128,7 +128,7 @@
 static int pch_power_options(struct udevice *pch)
 {
 	const void *blob = gd->fdt_blob;
-	int node = pch->of_offset;
+	int node = dev_of_offset(pch);
 	u8 reg8;
 	u16 reg16, pmbase;
 	u32 reg32;