devicetree: Discriminate device ops scan_bus()

Use of scan_static_bus() and tree traversals is somewhat convoluted.
Start cleaning this up by assigning each path type with separate
static scan_bus() function.

For ME, SMBus and LPC paths a bus cannot expose bridges, as those would
add to the number of encountered PCI buses.

Change-Id: I8bb11450516faad4fa33b8f69bce5b9978ec75e5
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/8534
Tested-by: build bot (Jenkins)
Reviewed-by: Timothy Pearson <tpearson@raptorengineeringinc.com>

src/southbridge/intel/ibexpeak/me.c

@@ -634,7 +634,6 @@ static struct device_operations device_ops = {
 	.set_resources		= pci_dev_set_resources,
 	.enable_resources	= pci_dev_enable_resources,
 	.init			= intel_me_init,
-	.scan_bus		= scan_static_bus,
 	.ops_pci		= &pci_ops,
 };