arm64: check for NULL bus on DEVICE_PATH_CPU_CLUSTER devices

If there are no devices underneath a device in a devicetree the
bus pointer in a struct device is NULL. Check for this condition
before proceeding in walking through the children devices.

BUG=chrome-os-partner:31761
BRANCH=None
TEST=Ran through coreboot w/o any devices under the cpu_cluster device.
     No more exceptions.

Change-Id: I891aeb36319dce67ce9e431156c85c74177c7ab7
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/217511
Reviewed-by: Tom Warren <twarren@nvidia.com>
Reviewed-by: Furquan Shaikh <furquan@chromium.org>

src/arch/arm64/cpu.c

@@ -299,6 +299,11 @@ void arch_initialize_cpus(device_t cluster, struct cpu_control_ops *cntrl_ops)
 	}
 
 	bus = cluster->link_list;
+
+	/* Check if no children under this device. */
+	if (bus == NULL)
+		return;
+
 	entry = prepare_secondary_cpu_startup();
 
 	/* Initialize the cpu_info structures. */