vexpress64.conf: enable CONFIG_PREEMPT to workaround boot failure

The workaround to disable CONFIG_CPU_IDLE doesn't work for all the models.
Let's use the other one.

Signed-off-by: Andrey Konovalov <andrey.konovalov@linaro.org>
diff --git a/linaro/configs/vexpress64.conf b/linaro/configs/vexpress64.conf
index e2b8115..9a6f6a7 100644
--- a/linaro/configs/vexpress64.conf
+++ b/linaro/configs/vexpress64.conf
@@ -2,9 +2,9 @@
 CONFIG_SMP=y
 CONFIG_NR_CPUS=8
 CONFIG_HOTPLUG_CPU=y
+CONFIG_PREEMPT=y
 CONFIG_CMDLINE="console=ttyAMA0"
 CONFIG_COMPAT=y
-# CONFIG_CPU_IDLE is not set
 CONFIG_SMC91X=y
 CONFIG_INPUT_EVDEV=y
 CONFIG_SERIO_AMBAKMI=y
@@ -32,3 +32,4 @@
 CONFIG_VIRTIO_MMIO=y
 CONFIG_REGULATOR=y
 CONFIG_REGULATOR_FIXED_VOLTAGE=y
+# CONFIG_DEBUG_PREEMPT is not set